With the introduction of LAMMPS_UPDATE, version.h is no longer a single line
file. With this change the CMake utility will only process the LAMMPS_VERSION
line. Fixes issue #3038
The function implementation was incomplete for handling the case
of multiple files. It would generate wrong C code in case more than
one files were specified. Since we don't have this use case, it's
safe to only leave the single input file implementation.
Closes issue #2294
