Work out pathname for gcc fixincludes headers via "-print-libgcc-file-name" and

Work out pathname for gcc fixincludes headers via "-print-libgcc-file-name" and
substitutions in depgen.sed at "make depend" time, and emit patterns into
depfix2.sed to strip them out of dependencies, instead of hard-coding pathnames
for Athena gcc installations we know about in depfix.sed.

Verify that gcc is being used, before trying to rebuild dependencies.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@14861 dc483132-0cff-0310-8789-dd5450dbe970