[krbdev.mit.edu #3011] krb5-config should never return -I/usr/include

[Public Submitter via RT]

Adding -I/usr/include to CFLAGS will cause recent gcc's to warn, which
can confuse configure scripts.  Worse, it reorders gcc's header search
path in a way that can potentially break things (by searching the system
include files before gcc's own overrides, for instance).

The attached patch changes krb5-config to never return -I/usr/include by
checking includedir against /usr/include before responding to --cflags.
 This is the same approach taken by the *-config scripts of many other
packages.