Avoid using function calls in OpenMP directives

The direct calls to gmx_omp_nthreads_get() that were
included in some OpenMP pragmas caused memory
corruption and later segfaults on PGI compilers. This
is likely a compiler bug, but we can work around it
by assigning the function return value to a variable
that we use in the pragma.

Such variables are unused when OpenMP is not in use, which might
offend some compiler some time, so adding a gmx_unused attribute is
useful. However, uncrustify needs to be taught about our custom
attributes, which is also done here.

Change-Id: I3b482bdc2401b40a043975ffd4a741f65efd0cfc