Make user-specified cflags take precedence over cc-rs flags

[madsmtm]

When the user uses .cflag/.cxxflag/.asmflag, we want those flags to take precedence over the default flags that cc-rs adds.

[madsmtm]

The nightly error is rust-lang/rust#136086, will be fixed by rust-lang/rust#136098.

[jyn514]

for posterity, the full error was

   Compiling test-crate v0.1.0 (/Users/runner/work/cmake-rs/cmake-rs/test-crate)
error: linker stderr: ld: search path '/Users/runner/work/cmake-rs/cmake-rs/test-crate/target/debug/build/libz-sys-ae8c893dbe111c55/out/lib64' not found
       ld: object file (/Users/runner/work/cmake-rs/cmake-rs/test-crate/target/debug/deps/liblibz_sys-297f35bb33641431.rlib[31](zutil.c.o)) was built for newer 'macOS' version (14.5) than being linked (11.0)
  |
  = note: `-D linker-messages` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(linker_messages)]`

(CI logs go away after about 3 months)

[jyn514]

note that i do not plan to silence the "search path" not found error, so that will eventually come back unless someone makes a convincing case why it shouldn't.