added gmx_is_{single,double}_precision

* allows easy detection of the precision for cmake, autotools
  without parsing the output of gmx_print_version_info (no
  cross-compile support) or the output of strings command (unix
  only)
* linking against libgmx with/out -DGMX_DOUBLE will lead to
  unpredictable segfaults

Change-Id: I472f10ae374a1f42c94c55e156b53f8905bdf098