how to select a particular gcc-toolchain in clang?

Clang automatically selects the gcc-version with the highest version:

$ clang++ -v main.cppclang version 3.8.1-12 (tags/RELEASE_381/final)Target: x86_64-pc-linux-gnuThread model: posixInstalledDir: /usr/binFound candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9.4Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.1Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.2.0Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.0.1Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.4Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.1Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.2.0Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.0.1Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.0.1

how can i force clang to use a different gcc installation, say 5.4.1 ?

i tried to call clang with --gcc-toolchain="/usr/lib/gcc/x86_64-linux-gnu/5.4.1" but without success.