GitLab

Bug: https://github.com/android-ndk/ndk/issues/222 : CMake toolchain
file's `CMAKE_CXX_COMPILER_ID_RUN=true` breaks compiler feature
detection
Bug: https://github.com/android-ndk/ndk/issues/253 : cmake toolchain
file does not set CMAKE_CXX_COMPILER_VERSION
Bug: https://gitlab.kitware.com/cmake/cmake/issues/16587 : Adding
"-march=" to flags breaks determining compiler ID when cross-compiling
with Clang

The current comment reads:

> Clang can fail to compile if CMake doesn't correctly supply the target
> and external toolchain, but to do so, CMake needs to already know
> that the compiler is clang. Tell CMake that the compiler is really
> clang, but don't use CMakeForceCompiler, since we still want
> compile checks. We only want to skip the compiler ID detection
> step.

The issue arises in CMakeDetermineCompilerId.cmake, which passes along
user-specified flags like `-march=armv5te` but not the specified
compiler target. On Clang, this fails, since the validity of `-march`
depends on the current target.

I've filed a bug for CMake to fix this (allow determining the compiler
ID without using user flags). In the meantime, I think a more robust
solution than overriding CMAKE_CXX_COMPILER_ID_RUN is to run the
compiler ID checks before passing along any flags. The
CMakeDetermineCCompiler.cmake and CMakeDetermineCXXCompiler.cmake
modules are not marked for internal-use-only, and this method takes a
more proactive approach to the problem.

Change-Id: I2c250ebc2999b3f251417a9e3730b5e093e446d1