Fix inability to find gmp.h when building concurrently

Authored by mpickering on Jul 22 2016, 7:03 AM.

Description

Fix inability to find gmp.h when building concurrently

There were situations when building concurrently when we would
request gmp.h before it had been built (or copied).

This was occuring when we generated the list of dependents for
the c files in the integer-gmp folder. Thus, when generating the
dependents for this library we now require gmp.h.