Improve handling of LAMMPS integer size defines in library.h

[akohlmey]

Summary

When including library.h when using the LAMMPS library interface, we need to set one of -DLAMMPS_BIGBIG, -DLAMMPS_SMALLBIG, or -DLAMMPS_SMALLSMALL. This should be the same as was used when compiling the library interface, but until we generate a file lmpconfig.h (similar to how we use lmpgitversion.h) that contains this information and include it, we have to depend on the user setting the correct value. For now we assume -DLAMMPS_SMALLBIG if no define is set by the user.

This PR also changes the incorrect use of #elif that was recently added.

Related Issues

Fixes #1791

Author(s)

Axel Kohlmeyer (Temple U)

Licensing

By submitting this pull request, I agree, that my contribution will be included in LAMMPS and redistributed under either the GNU General Public License version 2 (GPL v2) or the GNU Lesser General Public License version 2.1 (LGPL v2.1).

Post Submission Checklist

• The feature or features in this pull request is complete
• Licensing information is complete
• Corresponding author information is complete
• The source code follows the LAMMPS formatting guidelines
• The feature has been verified to work with the conventional build system
• The feature has been verified to work with the CMake based build system