smarter -Wunused-variable

[Jack Howarth]

    While weeding through the warnings from a build of xplor-nih with -Wall
I noticed slews of unused variable warnings. However xplor-nih uses lots
of COMMON blocks. It would be helpful if gfortran could issue warnings
for only variables that are not part of COMMON blocks. Currently I have
to manually work through the list and eliminate all of those which happen
to be in a COMMON block.
         Jack