Ensure we don't #define constants before system headers

For whatever reason, some older libc (e.g. as found on BG/Q login
nodes) live life such that they unilaterally #define some of the same
M_* constants that we do. If we ever #include <math.h> after
"gromacs/math/utilities.h" then those constants can differ and cause
compilation errors, because we've double-crossed our own #ifndef
attempts.

Change-Id: I4bd6d80e87489cc6f616ce0fb4260a0ac4f9e111