[krbdev.mit.edu #2923] provide autoconf macro for using krb5-config

[Tom Yu via RT]

Dealing with the RPATH issue would involve altering the krb5-config
script a little more versatile about how it extracts various things
relating to libraries.  Any ideas on what to call the various new
options to krb5-config?  Do we also want to separate CFLAGS from
CPPFLAGS, etc.?  That might open up a huge can of worms relating to
thread support.