Set CMAKE_SYSTEM_NAME globally

[matthewbauer]

Issue description

Tracking issue for setting CMAKE_SYSTEM_NAME for proper CMake support for cross compilation. Most likely we will need to create a cmake wrapper to handle this.

See discussion in #46773.

[matthewbauer]

Closed in favor of #25477

[mcmtroffaes]

Just to note here that, for mingw builds, setting CMAKE_CROSSCOMPILING but not CMAKE_SYSTEM_NAME may not be enough for cmake to figure out that the target system is Windows. So, at least for mingw, it seems that CMAKE_SYSTEM_NAME still needs to be set globally.

For reference, here's the output for building brotli with CMAKE_CROSSCOMPILING=True but without CMAKE_SYSTEM_NAME=Windows:

building '/nix/store/wm1n4yvpldz9rzam8vxca1ix6g9x01x6-brotli-1.0.6-x86_64-pc-mingw32.drv'...
unpacking sources
unpacking source archive /nix/store/3s7194w20rijmldh64riwi8pz12c641i-source
source root is source
patching sources
configuring
fixing cmake files...
cmake flags: -DCMAKE_BUILD_TYPE=Release -DCMAKE_SKIP_BUILD_RPATH=ON -DCMAKE_INSTALL_INCLUDEDIR=/nix/store/8h2670qh94gi0sa02fzqq3rrczcwhbrg-brotli-1.0.6-x86_64-pc-mingw32-dev/include -DCMAKE_INSTALL_LIBDIR=/nix/store/05ysmxinr24zq8pswid9ygrmaj0pi3m3-brotli-1.0.6-x86_64-pc-mingw32-lib/lib -DCMAKE_INSTALL_NAME_DIR=/nix/store/05ysmxinr24zq8pswid9ygrmaj0pi3m3-brotli-1.0.6-x86_64-pc-mingw32-lib/lib -DCMAKE_POLICY_DEFAULT_CMP0025=NEW -DCMAKE_OSX_DEPLOYMENT_TARGET= -DCMAKE_OSX_SYSROOT= -DCMAKE_FIND_FRAMEWORK=last -DCMAKE_STRIP=/nix/store/b30c9sig3nkr8k2vi2j2ia9dylxy0grr-x86_64-pc-mingw32-binutils-2.30/bin/x86_64-pc-mingw32-strip -DCMAKE_RANLIB=/nix/store/b30c9sig3nkr8k2vi2j2ia9dylxy0grr-x86_64-pc-mingw32-binutils-2.30/bin/x86_64-pc-mingw32-ranlib -DCMAKE_AR=/nix/store/b30c9sig3nkr8k2vi2j2ia9dylxy0grr-x86_64-pc-mingw32-binutils-2.30/bin/x86_64-pc-mingw32-ar -DCMAKE_C_COMPILER=x86_64-pc-mingw32-gcc -DCMAKE_CXX_COMPILER=x86_64-pc-mingw32-g++ -DCMAKE_INSTALL_PREFIX=/nix/store/4s8q9ci827wb9isp25rjvrfxhzwl82gz-brotli-1.0.6-x86_64-pc-mingw32 -DCMAKE_CROSSCOMPILING=True 
-- The C compiler identification is GNU 7.3.0
-- Check for working C compiler: /nix/store/m5csa3c3ma785c6d2ahap0v8d4vzjx1d-x86_64-pc-mingw32-stage-final-gcc-debug-wrapper-7.3.0/bin/x86_64-pc-mingw32-gcc
-- Check for working C compiler: /nix/store/m5csa3c3ma785c6d2ahap0v8d4vzjx1d-x86_64-pc-mingw32-stage-final-gcc-debug-wrapper-7.3.0/bin/x86_64-pc-mingw32-gcc -- broken
CMake Error at /nix/store/gwynihr81f3x5dcqdijk7p8wqgklqbb7-cmake-3.12.1/share/cmake-3.12/Modules/CMakeTestCCompiler.cmake:52 (message):
  The C compiler

    "/nix/store/m5csa3c3ma785c6d2ahap0v8d4vzjx1d-x86_64-pc-mingw32-stage-final-gcc-debug-wrapper-7.3.0/bin/x86_64-pc-mingw32-gcc"

  is not able to compile a simple test program.

  It fails with the following output:

    Change Dir: /tmp/nix-build-brotli-1.0.6-x86_64-pc-mingw32.drv-0/source/build/CMakeFiles/CMakeTmp
    
    Run Build Command:"/nix/store/ql3azyviwyc6w73h6prk1j2945skfapf-gnumake-4.2.1/bin/make" "cmTC_f6e44/fast"
    /nix/store/ql3azyviwyc6w73h6prk1j2945skfapf-gnumake-4.2.1/bin/make -f CMakeFiles/cmTC_f6e44.dir/build.make CMakeFiles/cmTC_f6e44.dir/build
    make[1]: Entering directory '/tmp/nix-build-brotli-1.0.6-x86_64-pc-mingw32.drv-0/source/build/CMakeFiles/CMakeTmp'
    Building C object CMakeFiles/cmTC_f6e44.dir/testCCompiler.c.o
    /nix/store/m5csa3c3ma785c6d2ahap0v8d4vzjx1d-x86_64-pc-mingw32-stage-final-gcc-debug-wrapper-7.3.0/bin/x86_64-pc-mingw32-gcc    -o CMakeFiles/cmTC_f6e44.dir/testCCompiler.c.o   -c /tmp/nix-build-brotli-1.0.6-x86_64-pc-mingw32.drv-0/source/build/CMakeFiles/CMakeTmp/testCCompiler.c
    Linking C executable cmTC_f6e44
    /nix/store/gwynihr81f3x5dcqdijk7p8wqgklqbb7-cmake-3.12.1/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f6e44.dir/link.txt --verbose=1
    /nix/store/m5csa3c3ma785c6d2ahap0v8d4vzjx1d-x86_64-pc-mingw32-stage-final-gcc-debug-wrapper-7.3.0/bin/x86_64-pc-mingw32-gcc      -rdynamic CMakeFiles/cmTC_f6e44.dir/testCCompiler.c.o  -o cmTC_f6e44 
    x86_64-pc-mingw32-gcc: error: unrecognized command line option '-rdynamic'
    make[1]: *** [CMakeFiles/cmTC_f6e44.dir/build.make:87: cmTC_f6e44] Error 1
    make[1]: Leaving directory '/tmp/nix-build-brotli-1.0.6-x86_64-pc-mingw32.drv-0/source/build/CMakeFiles/CMakeTmp'
    make: *** [Makefile:121: cmTC_f6e44/fast] Error 2