Polish checks for working Kerberos library

[yadij]

These checks test whether the library chosen is able to be used without any specific features Squid may have workarounds for.

Take advantage of AC_CACHE better to enable reentrant macros instead of requiring order-specific calls and update to use the simpler AC_LANG_PROGRAM macro. Also fix several cases of missing support for cross-compile.

Now that they are reentrant shuffle library-specific brokenness checks inside SQUID_CHECK_WORKING_KRB5 macro and set the relevant AC_DEFINE as-needed instead of in each caller.

Fix bug where the test for max_skew was never being done. Test if necessary to define a missing krb5_get_max_time_skew() function.