Refactor lammps formula and fix OpenMP implementation

This includes changes to the following:

1. Single, unified library build function: more maintainable and easier
   to read
2. Support for fortran compiler other than gfortran (use FC)
3. Better enforcement of clang
4. Fix OpenMP installation
  * depends on 'homebrew/dupes/gcc' because some functions in gcc42 omp
    fail to build
5. Patch for a bug introduced in the 2013.02.11 version of LAMMPS
   user-omp
6. Roll back incorrect version-number explanation. It turns out that it
   is 'days' after all
7. add `voro++` dependency