Remove GMX_DOUBLE from config.h

It is not feasible for us to remove GMX_DOUBLE from installed headers
any time in the foreseeable future.  And we cannot define it in the
installed headers, since we want to use the same set of headers for both
single- and double-precision binaries.

This means that code calling Gromacs will always need to define it
somehow external to the headers, and ensure that it is always there.
Now, our own build system also uses the same approach and defines it on
the compiler command line.  Another alternative would be to have a
gmxpre.h header that would get included into each and every source file.
We may need such a header for some other purposes, but it should be much
more static than the current config.h...

This removes one of the main obstacles of getting rid of config.h
dependencies throughout the code.

Change-Id: I20cb66d5e58c56212a3b23f8bdea8dd6b9eefb2c