Enable new SQL generator on Mac

[Hoverbear]

We whoops'd in #165 and the PR didn't quite work as expected on Mac!

This PR fixes it by using Mac specific flags as determined by @eeeebbbbrrrr .

[eeeebbbbrrrr]

a cargo pgx install on the strings example worked fine on my Mac, however, a release build failed:

$ cargo pgx install --release
building extension with features `pg12`
"cargo" "build" "--release" "--features" "pg12" "--no-default-features"
   Compiling libc v0.2.102
   Compiling autocfg v1.0.1
   Compiling proc-macro2 v1.0.28
   Compiling syn v1.0.76
   Compiling unicode-xid v0.2.2
   Compiling version_check v0.9.3
   Compiling cfg-if v1.0.0
   Compiling memchr v2.4.0
   Compiling serde_derive v1.0.130
   Compiling serde v1.0.130
   Compiling log v0.4.14
   Compiling lazy_static v1.4.0
   Compiling ryu v1.0.5
   Compiling serde_json v1.0.68
   Compiling getrandom v0.1.16
   Compiling crc32fast v1.2.1
   Compiling regex-syntax v0.6.25
   Compiling cc v1.0.69
   Compiling eyre v0.6.5
   Compiling once_cell v1.8.0
   Compiling tinyvec_macros v0.1.0
   Compiling matches v0.1.8
   Compiling pin-project-lite v0.2.7
   Compiling adler v1.0.2
   Compiling crossbeam-utils v0.8.5
   Compiling itoa v0.4.7
   Compiling percent-encoding v2.1.0
   Compiling radium v0.5.3
   Compiling smallvec v1.6.1
   Compiling ansi_term v0.12.1
   Compiling glob v0.3.0
   Compiling unicode-width v0.1.8
   Compiling ppv-lite86 v0.2.10
   Compiling crossbeam-epoch v0.9.5
   Compiling gimli v0.25.0
   Compiling byteorder v1.4.3
   Compiling vec_map v0.8.2
   Compiling tap v1.0.1
   Compiling scopeguard v1.1.0
   Compiling rustc-demangle v0.1.20
   Compiling funty v1.1.0
   Compiling ucd-trie v0.1.3
   Compiling strsim v0.8.0
   Compiling owo-colors v1.3.0
   Compiling typenum v1.13.0
   Compiling mime v0.3.16
   Compiling wyz v0.2.0
   Compiling rayon-core v1.9.1
   Compiling bitflags v1.3.2
   Compiling ansi_term v0.11.0
   Compiling indenter v0.3.3
   Compiling xml-rs v0.8.4
   Compiling httpdate v0.3.2
   Compiling base64 v0.11.0
   Compiling humantime v2.1.0
   Compiling bindgen v0.59.1
   Compiling unescape v0.1.0
   Compiling termcolor v1.1.2
   Compiling proc-macro-hack v0.5.19
   Compiling shlex v1.0.0
   Compiling lazycell v1.3.0
   Compiling rustc-hash v1.1.0
   Compiling either v1.6.1
   Compiling peeking_take_while v0.1.2
   Compiling const_fn v0.4.8
   Compiling hashbrown v0.11.2
   Compiling heapless v0.6.1
   Compiling stable_deref_trait v1.2.0
   Compiling half v1.7.1
   Compiling fixedbitset v0.2.0
   Compiling seahash v4.1.0
   Compiling libloading v0.7.0
   Compiling tracing-core v0.1.18
   Compiling sharded-slab v0.1.3
   Compiling tinyvec v1.3.1
   Compiling unicode-bidi v0.3.5
   Compiling thread_local v1.1.3
   Compiling unicase v2.6.0
   Compiling nom v6.1.2
   Compiling standback v0.2.17
   Compiling generic-array v0.14.4
   Compiling time v0.2.27
   Compiling num-traits v0.2.14
   Compiling num-integer v0.1.44
   Compiling miniz_oxide v0.4.4
   Compiling memoffset v0.6.4
   Compiling rayon v1.5.1
   Compiling indexmap v1.7.0
   Compiling form_urlencoded v1.0.1
   Compiling textwrap v0.11.0
   Compiling build-deps v0.1.4
   Compiling pest v2.1.3
   Compiling clang-sys v1.2.0
   Compiling backtrace v0.3.61
   Compiling hash32 v0.1.1
   Compiling regex-automata v0.1.10
   Compiling addr2line v0.16.0
   Compiling unicode-normalization v0.1.19
   Compiling tracing-log v0.1.2
   Compiling aho-corasick v0.7.18
   Compiling object v0.26.0
   Compiling semver-parser v0.10.2
   Compiling matchers v0.0.1
   Compiling atty v0.2.14
   Compiling socks v0.3.3
   Compiling dirs-sys v0.3.6
   Compiling getrandom v0.2.3
   Compiling quote v1.0.9
   Compiling num_cpus v1.13.0
   Compiling which v3.1.1
   Compiling crossbeam-channel v0.5.1
   Compiling bitvec v0.19.5
   Compiling generic-array v0.13.3
   Compiling generic-array v0.12.4
   Compiling rand_core v0.5.1
   Compiling clap v2.33.3
   Compiling colored v2.0.0
   Compiling dirs v3.0.2
   Compiling idna v0.2.3
   Compiling regex v1.5.4
   Compiling semver v0.11.0
   Compiling uuid v0.8.2
   Compiling mime_guess v2.0.3
   Compiling flate2 v1.0.20
   Compiling petgraph v0.5.1
   Compiling rand_chacha v0.2.2
   Compiling as-slice v0.1.5
   Compiling rustc_version v0.3.3
   Compiling url v2.2.2
   Compiling env_logger v0.8.4
   Compiling crossbeam-deque v0.8.1
   Compiling chrono v0.4.19
   Compiling rand v0.7.3
   Compiling atomic-traits v0.3.0
   Compiling env_proxy v0.4.1
   Compiling cexpr v0.5.0
   Compiling rttp_client v0.1.0
   Compiling tracing-attributes v0.1.15
   Compiling thiserror-impl v1.0.26
   Compiling time-macros-impl v0.1.2
   Compiling enum-primitive-derive v0.2.1
   Compiling thiserror v1.0.26
   Compiling tracing v0.1.26
   Compiling time-macros v0.1.1
   Compiling tracing-serde v0.1.2
   Compiling serde-xml-rs v0.5.1
   Compiling toml v0.5.8
   Compiling serde_cbor v0.11.2
   Compiling tracing-subscriber v0.2.20
   Compiling proc-macro-crate v1.0.0
   Compiling tracing-error v0.1.2
   Compiling color-spantrace v0.1.6
   Compiling color-eyre v0.5.11
   Compiling pgx-utils v0.1.22 (/Users/e_ridge/_work/pgx/pgx-utils)
   Compiling pgx-pg-sys v0.1.22 (/Users/e_ridge/_work/pgx/pgx-pg-sys)
   Compiling pgx-macros v0.1.22 (/Users/e_ridge/_work/pgx/pgx-macros)
   Compiling pgx v0.1.22 (/Users/e_ridge/_work/pgx/pgx)
   Compiling strings v0.1.0 (/Users/e_ridge/_work/pgx/pgx-examples/strings)
    Finished release [optimized] target(s) in 2m 04s

installing extension
     Copying control file to `/Users/zdb/pg12/share/postgresql/extension/strings.control`
     Copying shared library to `/Users/zdb/pg12/lib/postgresql/strings.so`
    Building SQL generator with features `pg12`
"cargo" "build" "--bin" "sql-generator" "--release" "--features" "pg12" "--no-default-features"
   Compiling strings v0.1.0 (/Users/e_ridge/_work/pgx/pgx-examples/strings)
    Finished release [optimized] target(s) in 1.97s
 Discovering SQL entities
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', cargo-pgx/src/commands/install.rs:151:6
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It doesn't say which file is missing, so it's a little tough to debug...

[Hoverbear]

@zombodb Thanks!!! We fixed this together! :) Could you confirm this works in debug and releas eon Mac now?