Added two #defines that state the gromacs API + program version.

Added two #defines that state the gromacs API + program version.

Added two #defines in a new CMake-generated header file version.h:
GMX_API_VERSION, and GMX_VERSION so third-party tools can provide some
backward compatibility through

  #if !defined(GMX_API_VERSION) || (GMX_API_VERSION < 40600)
    ....  <do 4.5 specific stuff>
  #elif (GMX_API_VERSION < 40700)
    ....  <do 4.6 specific stuff>
  #endif

For now (pre-5.0), version.h is included in typedefs.h, which should
already be included in most gromacs-compatible code.

The API version is a separate CMake variable that currently gets its
value from the major+minor release version.

Change-Id: I9898cf378216d63c1456ce6566a2e42774fc1a0c