A lot of those warnings are spawned on my system:

configure.in:993: warning: AC_CACHE_VAL(libunixodbc_usable, ...): suspicious cache-id, must contain _cv to be cached
../../lib/autoconf/general.m4:2018: AC_CACHE_VAL is expanded from...
../../lib/autoconf/general.m4:2039: AC_CACHE_CHECK is expanded from...
m4/libunixodbc.m4:20: LIBUNIXODBC_CHECK_CONFIG is expanded from...
configure.in:993: the top level

The fix is trivial, change in m4/libunixodbc.m4 every (5) _libunixodbc_usable to _cv_libunixodbc_usable.

Grtz,
Steven