Add GCC compilers (C, C++ and Fortran)

OSX Mavericks:
```
[jtang@awesome-o bin]$ otool -L gfortran
gfortran:
/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1197.1.1)
[jtang@awesome-o bin]$ otool -L x86_64-apple-darwin13.3.0-gfortran
x86_64-apple-darwin13.3.0-gfortran:
/usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1197.1.1)
[jtang@awesome-o bin]$ file gfortran
gfortran: Mach-O 64-bit executable x86_64
[jtang@awesome-o bin]$ pwd
/Users/jtang/develop/hashstack2/gcc/bin
[jtang@awesome-o bin]$ ./gfortran --version
GNU Fortran (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.

GNU Fortran comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of GNU Fortran
under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING

[jtang@awesome-o bin]$
```