Declare __STDC_*_MACROS in gmxpre.h

These free up code that needs, e.g., GMX_INT64_MAX from hard-to-satisfy
constraints on the include order.  More details in the comments in
gmxpre.h.

Also clarify our requirements for C99 support in the install guide.

Change-Id: I371c3a79244340764b320ae36183dd8926fe0f94