Removed gmx_header_config.h

Removed gmx_header_config.h

Now we no longer install a header just to get platform
independence for functionality that is not actually
part of the the installed API.

Moved DIR_SEPARATOR into its own non-installed header.

Moved snprintf MSVC-workaround define into its own non-installed
header.

Moved GMX_NATIVE_WINDOWS definition to config.h

Removed checks related to the use of gmx_header_config.h

Refs #950, #1454

Change-Id: I6aebcddd6772dfa82bf1214c6ed42f9da6ac22e0