Missing unified header include paths lead in broken builds when having native dependencies

[agorgl]

Building android apps from crates that have native dependencies currently not possible, due to the structure of the toolchains inside Android NDK that has the standard library includes in a separate folder from the toolchain sysroot. For example if i try to use backtrace-rs dependency in my project i get this during build:

error: failed to run custom build command for `backtrace-sys v0.1.24`
process didn't exit successfully: `/home/theartist/Workdir/Charge/target/debug/build/backtrace-sys-60c2d4f631e66e45/build-script-build` (exit code: 101)
--- stdout
TARGET = Some("arm-linux-androideabi")
OPT_LEVEL = Some("0")
HOST = Some("x86_64-unknown-linux-gnu")
CC_arm-linux-androideabi = Some("/opt/android-ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc")
CFLAGS_arm-linux-androideabi = Some("--sysroot /opt/android-ndk/platforms/android-28/arch-arm")
DEBUG = Some("true")
running: "/opt/android-ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "--sysroot" "/opt/android-ndk/platforms/android-28/arch-arm" "-g" "-fno-omit-frame-pointer" "-I" "src/libbacktrace" "-I" "/home/theartist/Workdir/Charge/target/arm-linux-androideabi/debug/build/backtrace-sys-f9d57695cf709455/out" "-fvisibility=hidden" "-DBACKTRACE_ELF_SIZE=32" "-DBACKTRACE_SUPPORTED=1" "-DBACKTRACE_USES_MALLOC=1" "-DBACKTRACE_SUPPORTS_THREADS=0" "-DBACKTRACE_SUPPORTS_DATA=0" "-D_GNU_SOURCE=1" "-D_LARGE_FILES=1" "-Dbacktrace_full=__rbt_backtrace_full" "-Dbacktrace_dwarf_add=__rbt_backtrace_dwarf_add" "-Dbacktrace_initialize=__rbt_backtrace_initialize" "-Dbacktrace_pcinfo=__rbt_backtrace_pcinfo" "-Dbacktrace_syminfo=__rbt_backtrace_syminfo" "-Dbacktrace_get_view=__rbt_backtrace_get_view" "-Dbacktrace_release_view=__rbt_backtrace_release_view" "-Dbacktrace_alloc=__rbt_backtrace_alloc" "-Dbacktrace_free=__rbt_backtrace_free" "-Dbacktrace_vector_finish=__rbt_backtrace_vector_finish" "-Dbacktrace_vector_grow=__rbt_backtrace_vector_grow" "-Dbacktrace_vector_release=__rbt_backtrace_vector_release" "-Dbacktrace_close=__rbt_backtrace_close" "-Dbacktrace_open=__rbt_backtrace_open" "-Dbacktrace_print=__rbt_backtrace_print" "-Dbacktrace_simple=__rbt_backtrace_simple" "-Dbacktrace_qsort=__rbt_backtrace_qsort" "-Dbacktrace_create_state=__rbt_backtrace_create_state" "-Dbacktrace_uncompress_zdebug=__rbt_backtrace_uncompress_zdebug" "-o" "/home/theartist/Workdir/Charge/target/arm-linux-androideabi/debug/build/backtrace-sys-f9d57695cf709455/out/src/libbacktrace/alloc.o" "-c" "src/libbacktrace/alloc.c"
cargo:warning=src/libbacktrace/alloc.c:35:19: fatal error: errno.h: No such file or directory
cargo:warning= #include <errno.h>
cargo:warning=                   ^
cargo:warning=compilation terminated.
exit code: 1

--- stderr
thread 'main' panicked at '

Internal error occurred: Command "/opt/android-ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-gcc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "--sysroot" "/opt/android-ndk/platforms/android-28/arch-arm" "-g" "-fno-omit-frame-pointer" "-I" "src/libbacktrace" "-I" "/home/theartist/Workdir/Charge/target/arm-linux-androideabi/debug/build/backtrace-sys-f9d57695cf709455/out" "-fvisibility=hidden" "-DBACKTRACE_ELF_SIZE=32" "-DBACKTRACE_SUPPORTED=1" "-DBACKTRACE_USES_MALLOC=1" "-DBACKTRACE_SUPPORTS_THREADS=0" "-DBACKTRACE_SUPPORTS_DATA=0" "-D_GNU_SOURCE=1" "-D_LARGE_FILES=1" "-Dbacktrace_full=__rbt_backtrace_full" "-Dbacktrace_dwarf_add=__rbt_backtrace_dwarf_add" "-Dbacktrace_initialize=__rbt_backtrace_initialize" "-Dbacktrace_pcinfo=__rbt_backtrace_pcinfo" "-Dbacktrace_syminfo=__rbt_backtrace_syminfo" "-Dbacktrace_get_view=__rbt_backtrace_get_view" "-Dbacktrace_release_view=__rbt_backtrace_release_view" "-Dbacktrace_alloc=__rbt_backtrace_alloc" "-Dbacktrace_free=__rbt_backtrace_free" "-Dbacktrace_vector_finish=__rbt_backtrace_vector_finish" "-Dbacktrace_vector_grow=__rbt_backtrace_vector_grow" "-Dbacktrace_vector_release=__rbt_backtrace_vector_release" "-Dbacktrace_close=__rbt_backtrace_close" "-Dbacktrace_open=__rbt_backtrace_open" "-Dbacktrace_print=__rbt_backtrace_print" "-Dbacktrace_simple=__rbt_backtrace_simple" "-Dbacktrace_qsort=__rbt_backtrace_qsort" "-Dbacktrace_create_state=__rbt_backtrace_create_state" "-Dbacktrace_uncompress_zdebug=__rbt_backtrace_uncompress_zdebug" "-o" "/home/theartist/Workdir/Charge/target/arm-linux-androideabi/debug/build/backtrace-sys-f9d57695cf709455/out/src/libbacktrace/alloc.o" "-c" "src/libbacktrace/alloc.c" with args "arm-linux-androideabi-gcc" did not execute successfully (status code exit code: 1).

', /home/theartist/.cargo/registry/src/github.aaakk.us.kg-1ecc6299db9ec823/cc-1.0.25/src/lib.rs:2260:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Build failed, waiting for other jobs to finish...
error: build failed

After lots of inspection, i found out that the exported variables here:
https://github.com/tomaka/android-rs-glue/blob/master/cargo-apk/src/ops/build.rs#L197 are picked up and used by the cc crate that is used to build the native components of backtrace-rs (backtrace-sys).
NDK though, has the standard include headers in different directory than the platform specific one that is passed as a parameter to --sysroot.

Proposed solution is to add to the set env vars the -isysroot flag that points to the $NDK_HOME/sysroot dir that contains the unified headers path, along with a -isystem flag that points to the architecture specific includes (needed by the unified headers) that reside here: $NDK_HOME/sysroot/usr/include/<target_arch>

I'm prepping a PR for this.

EDIT:
Added #186