build clang from source using specific gcc toolchain

Question

I'm building clang from source, but using a gcc 7.2 build that doesn't live in the normal spot. I want the resulting clang++ binary to use this toolchain by default. I tried to do:

$ export GCC_PREFIX=/path/to/gcc/7.2
$ mkdir -p build
$ cd build
$ cmake -G Ninja -DCMAKE_C_COMPILER=${GCC_PREFIX}/bin/gcc \
     -DCMAKE_CXX_COMPILER=${GCC_PREFIX}/bin/g++ \
     -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLLVM_TARGETS_TO_BUILD=X86 ..

But the resulting build just looks in /usr/include/c++/4.8.5 for standard library includes instead of /path/to/gcc/7.2/include like I wanted it to (and, as the path suggests, 4.8.5 is a bit old). Is there a way to build clang such that the resulting binary will search the path I want for its include directory?

Note, I know I run the produced binary with --gcc-toolchain=${GCC_PREFIX}. That works. What I want to do is build that into the binary so I don't have to type it every time.

Answer 1

Just found this. The correct way to do this is to provide GCC_INSTALL_PREFIX:

$ cmake -G Ninja -DCMAKE_C_COMPILER=${GCC_PREFIX}/bin/gcc \
     -DCMAKE_CXX_COMPILER=${GCC_PREFIX}/bin/g++ \

     -DGCC_INSTALL_PREFIX=${GCC_PREFIX} \

     -DCMAKE_BUILD_TYPE=RelWithDebInfo -DLLVM_TARGETS_TO_BUILD=X86 ..

That does the right thing. The resulting clang binary behaves as if you provided --gcc-toolchain=${GCC_PREFIX}, as desired.