gmxapi CMake toolchain

[eirrgang]

Life is simplest when the Python gmx package is built with the same compiler as libgromacs and libgmxapi, but this typically is a burden on the user. It might help for gromacs-gmxapi to export a CMake toolchain file that the CMakeLists.txt files in gmxapi and sample_restraint will look for to get the right compilers. On the other hand, if we miss something and environment variables still need to be set to find the right libc or some such thing then the errors could be more confusing. We could instead reimplement or reuse gromacs_check_compiler() from gromacs/share/cmake/gromacs/gromacs-config.cmake to make a fatal error with explicit documentation.