Further fixes in src/GNUmakefile

(1) Updated incorrect "front-page" comments about how to build
    the GEOS-Chem-Libraries

(2) Added variable GCC to allow the user to pick the GNU
    C/C++ compilers rather than the IFORT or PGI compilers.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>
Updated comments in README and src/GNUmakefile

README: Now point the reader to the wiki page:
http://wiki.seas.harvard.edu/geos-chem/index.php/Installing_libraries_for_GEOS-Chem

src/GNUmakefile: Updated comments that describe how to build
the libraries.  Added info about the GCC option.  Corrected
some typos as well.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>