remove the compiler's absolute path if it has one.

Modified Makefile so that objects are only rebuilt if the need to be.

Modified bg compiler generation script to remove compiler paths from bg
compilers and leave the absolute paths for gnu compilers.


git-svn-id: https://svn.mcs.anl.gov/repos/darshan/branches/trac5@317 3b7491f3-a168-0410-bf4b-c445ed680a29