m4: WARN FLAGS: Drop -Wenum-compare

My RHEL-6 gcc (4.4.7) complains a lot about it:
   cc1: warning: command line option "-Wenum-compare" is
   valid for C++/ObjC++ but not for C

For older gcc version (e.g. 4.4.7), man gcc says
-Wenum-compare is "C++ and Objective-C++ only".

For newer gcc (e.g. 4.9.2), man gcc says
"In C this warning is enabled by -Wall."

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