Detecting if a system installation has package enabled

[ExpHP]

Wonderful things were added to library.h in recent versions:

int lammps_config_has_package(char * package_name);
int lammps_config_package_count();
int lammps_config_package_name(int index, char * buffer, int max_size);

It would be awesome to use these somehow to validate that certain features have been enabled in a system installation of lammps. Like the exceptions feature, we could reject a system library missing these features and fall back to a source build.