Fixes and updates to BlueGene/Q support

Stopped hard-coding compilers, since these vary between sites. Moved
compiler suppressions to gmxCFlags to follow standard practice better.

Permitted use of software invsqrt, even though you don't want to use
it.

Suppressed several kinds of false-alarm warnings with whole-of-source
compiler suppressions. I would preferred to keep the suppressions as
file-level compiler flags, but the way we compile our list of source
files for libgromacs makes this inconvenient, and the compiler would
not accept the PBC-related suppression as a pragma, either. Build is
now finally free of warnings (so long as you don't use the built-in
LAPACK with bgclang-3.5). Restructured some helper functions that
return strings whose getters might throw, so that there is a return
value that will keep compilers happy in all cases.

Change-Id: I7158e165575934be2b76beee7c10f860ccf58b76