[boost-modular-build-helper] B2 ignores C/CXX/LDFLAGS when toolchain is not MSVC or GCC

[psfrolov]

Problem

In user-config.jam we have:

if "@VCPKG_PLATFORM_TOOLSET@" != "external"
{
    using msvc : : cl.exe
        :
        <setup>"@NOTHING_BAT@"
        @TOOLSET_OPTIONS@
        ;
}
else
{
    using gcc : 5.4.1 : @CMAKE_CXX_COMPILER@
        :
        <ranlib>@CMAKE_RANLIB@
        <archiver>@CMAKE_AR@
        @CXXFLAGS@
        @CFLAGS@
        @LDFLAGS@
        # MINGW here causes b2 to not run cygpath
        <flavor>mingw
        ;
}

That said if toolset is not msvc we are going else route with toolset set to gcc 54.1. This mostly works but have subtle side effects if compiler we are building with is not in fact GCC. For example, if we are building with Clang then using gcc makes B2 to ignore all the options in using block because clang is not gcc. Thus no compiler/linker options from Vcpkg code and user triplets are passed to build.

Proposed solution

1. Make toolset and version in user-config.jam configurable:

using @VCPKG_B2_COMPILER_ID@ : @CMAKE_CXX_COMPILER_VERSION@ : @CMAKE_CXX_COMPILER@

2. In CMakeLists.txt map CMake toolchain ID to B2 toolchain ID:

# Determine compiler ID to use in user-config.jam
# CMake compiler IDs: https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_COMPILER_ID.html
# B2 compiler IDs: https://boostorg.github.io/build/manual/master/index.html#bbv2.reference.tools.compilers
if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
    set(VCPKG_B2_COMPILER_ID gcc)
elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")  # AppleClang, Clang, XLClang
    set(VCPKG_B2_COMPILER_ID clang)
elseif(...)
    ...
else()
    # Either hard fail or fallback to GCC and issue a warning.
    # Hard fail is better because alternative is undefined behaviour.
endif()

I consider also adding -d 2 to b2 invocation to capture compiler and linker command lines in B2 logs so such kind of issues can be debugged more easly.

[vejmartin]

Possibly fixed by #18529.

[yurybura]

@PhoebeHui This issue has been fixed already. Could you close it?

[PhoebeHui]

@yurybura, thank you for confirming this issue!