Merge PR #1946 (Fix build issues in GCClassic w/ the custom mech)

This merge brings PR #1946 (Fix build issues in GCClassic with the
custom mechanism, by @yantosca) into the GEOS-Chem 14.2.1 development
stream.

This fixes several issues and omissions in the KPP/custom directory
that prevented building the custom mechanism.  Also the mechanism
definition files custom.kpp/fullchem.kpp and custom.eqn/fullchem.eqn
have been synchronized (which had not been done recently).

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>