We cannot depend on a fixed location for cl.exe in a taskcluster world.
We therefore need to make our build-clang.py script accomodate relative
path names for cc/cxx and assume those are binaries that should be
looked up on PATH.
We also need to modify the Linux build script so that the virtualenv is
used to look up the new 'which' package.
We need to rebuild clang with libc++ to get compatible headers for cross
builds. libc++abi is a dependency of libc++, as the build instructions
says [0].
[0] http://libcxx.llvm.org/docs/BuildingLibcxx.html
Instead, copy libgcc from the GCC used to build clang at each stage.
When passing --gcc-toolchain, the flag ends up appearing in the output
of llvm-config, and completely defeats the purpose of copying libgcc in
clang/lib/gcc.
Since build-clang.py requires a gcc_dir to be set, and we're using GCC
from there to build clang, we might as well copy libgcc from there
instead of building a fresh GCC. On the taskcluster job building clang,
GCC comes from a tooltool package that is already the result of
build-gcc anyways.
When passing -DCMAKE_C_COMPILER="gcc -flags" to CMake, ninja then tries
to literally call "gcc -flags", not "gcc" "-flags", and a "gcc -flags"
file obviously doesn't exist, so the build fails.
This fixes a regression from bug 1042132. Hopefully, this also doesn't
regress bug 1042132 itself.
