Should there be a few environment variable options for the build: USER_FFLAGS

[ekluzek]

In light of some of the build customization issues I found: #4972 #4971 for example, I wonder if the simple approach in the CIME/Tools/Makefile for environment variables should be expanded.

The current list of those variables are:

USER_SLIBS
USER_CPPDEFS
USER_INCLDIR
USER_CMAKE_MODULE_PATH
USER_CMAKE_OPTS
USER_MKDEPENDS_OPTS

The only one I would use would be USER_FFLAGS, but I could see USER_CFLAGS being useful.