Fix --disable-core-odbc-support

In commit ffc8e81b, tc ensured that
configure would abort if libodbc was not found. However this resulted
in the library check being done twice, as well as rendering
--disable-core-odbc-support ineffective. If libodbc was found, it
would enable core ODBC support regardless. This fix ensures the check
is only done once or not at all if core ODBC support is explicitly
disabled.
Signed-off-by: Travis Cross <tc@traviscross.com>

configure.in

@@ -404,12 +404,13 @@ AM_CONDITIONAL([WANT_DEBUG],[test "${enable_debug}" = "yes"])
 
 AC_ARG_ENABLE(core-odbc-support,     
 	[AS_HELP_STRING([--enable-core-odbc-support], [Compile with ODBC Support])],,[enable_core_odbc_support="no"])
-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]))
+  AX_LIB_ODBC
+  if test "x$ac_cv_found_odbc" = "xyes" ; then
+    enable_core_odbc_support="yes"
+  elif test "x$enable_core_odbc_support" = "xyes" ; then
+    AC_MSG_ERROR([no usable libodbc; please install unixodbc devel package or equivalent])
+  fi
 fi
 
 CHECK_LIBUUID