Fixed use of isfinite and _isfinite

Fixed use of isfinite and _isfinite

Implemented proper CMake tests for the above functionality.
Catered for MSVC idiosyncratic _finite() function. Removed
erroneous use of HAS_ISFINITE and HAS__ISFINITE.
Encapsulated the use of preprocessor directives into the new
gmx_isfinite() function so the programmer doesn't need to
know about the foregoing mess.

The previous versions resulted in unreachable code.

Change-Id: I7471388ed8f402c3f8b664e7846465de4b85f578