check for missing unixodbc-dev during configure

Previously if --enable-core-odbc-support was set but unixodbc-dev was
not available, we would fail during make rather during configure.

configure.in

@@ -391,6 +391,10 @@ AX_LIB_ODBC
 if test "$ac_cv_found_odbc" = "yes" ; then
   enable_core_odbc_support="yes"
 fi
+if test "x$enable_core_odbc_support" != "xno"; then
+  AC_CHECK_LIB([odbc], [SQLDisconnect],, AC_MSG_ERROR([no usable libodbc; please install unixodbc devel package or equivalent]))
+fi
+
 
 
 AC_ARG_ENABLE(timerfd-wrapper,