grompp now warns if macros in mdp “define” field are unused in topology

Macros defined in the mdp (with e.g. -DPOSRES) now cause a warning in grompp if they are not encountered while parsing the topology file

Issue 1975

Introduced CMake variable GMX_VERSION_STRING_OF_FORK

To help users and developers understand which version of GROMACS is being used, anybody providing a forked version of GROMACS shall set GMX_VERSION_STRING_OF_FORK in the source code (or if necessary when running CMake). It will then appear in the log file and users will know which version and fork of the code produced the result.