c++tools: Fix up c++tools for --with-gcc-major-version-only

c++tools: Fix up c++tools for --with-gcc-major-version-only

Seems c++tools doesn't honor --with-gcc-major-version-only.
Our distro uses that flag and so everything is installed in
/usr/lib/gcc/<target>/11/...
/usr/libexec/gcc/<target>/11/...
except
/usr/libexec/gcc/<target>/11.0.0/g++-mapper-server

The following patch should fix that.

2020-12-17  Jakub Jelinek  <jakub@redhat.com>

	* configure.ac: Add GCC_BASE_VER.
	* Makefile.in (version): Remove variable.
	(gcc_version): New variable.
	(libexecsubdir): Use $(gcc_version) instead of $(version).
	* configure: Regenerated.