Specifying CMAKE_ASM_COMPILER to /usr/bin/cc on macOS cause build failures

[fanzeyi]

I have seen an error similar to #95 while building grpc-sys. It looks like with cmake-rs 0.1.43 & 0.1.44, CMAKE_ASM_COMPILER will be explicitly specified to /usr/bin/cc, and for some reason this made CMake to not able to identify the assembler type. This subsequently caused incorrect compiler flag being generated.

From CMake's record, with cmake-rs == 0.1.44. The generated configration is:

running: "cmake" "/Users/fanzeyi/.cargo/registry/src/github.com-1ecc6299db9ec823/grpcio-sys-0.5.3/grpc" "-DgRPC_INSTALL=false" "-DgRPC_BUILD_CSHARP_EXT=false" "-DgRPC_BUILD_CODEGEN=false" "-DgRPC_BENCHMARK_PROVIDER=none" "-DgRPC_ZLIB_PROVIDER=package" "-DCMAKE_INSTALL_PREFIX=/Users/fanzeyi/GitHub/project/target/debug/build/grpcio-sys-2c04b4ba668021f3/out" "-DCMAKE_C_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_C_COMPILER=/usr/bin/cc" "-DCMAKE_CXX_FLAGS= -stdlib=libc++ -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_COMPILER=/usr/bin/c++" "-DCMAKE_ASM_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_ASM_COMPILER=/usr/bin/cc" "-DCMAKE_BUILD_TYPE=Debug"
-- The ASM compiler identification is unknown
-- Found assembler: /usr/bin/cc
-- Warning: Did not find file Compiler/-ASM
...

With cmake-rs == 0.1.42, CMake has:

running: "cmake" "/Users/fanzeyi/.cargo/registry/src/github.com-1ecc6299db9ec823/grpcio-sys-0.5.3/grpc" "-DgRPC_INSTALL=false" "-DgRPC_BUILD_CSHARP_EXT=false" "-DgRPC_BUILD_CODEGEN=false" "-DgRPC_BENCHMARK_PROVIDER=none" "-DgRPC_ZLIB_PROVIDER=package" "-DCMAKE_INSTALL_PREFIX=/Users/fanzeyi/GitHub/project/target/debug/build/grpcio-sys-1925f40788408223/out" "-DCMAKE_C_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_C_COMPILER=/usr/bin/cc" "-DCMAKE_CXX_FLAGS= -stdlib=libc++ -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_COMPILER=/usr/bin/c++" "-DCMAKE_BUILD_TYPE=Debug"
...
-- The ASM compiler identification is AppleClang
-- Found assembler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc
...

If you compare the CMake command executed, in 0.1.44 it is specifying -DCMAKE_ASM_COMPILER to /usr/bin/cc (at the very end) while 0.1.42 does not.

[fanzeyi]

Nevermind, I think this is a CMake bug. CMake should be able to detect the assembler correctly.