Fix configure ODBC logic

...and rewrite entire block for better clarity of purpose.

We might want to look more closely at the AX_LIB_ODBC macro as well.

This amends commit dfd591e9.

configure.in

@@ -402,15 +402,17 @@ AM_CONDITIONAL([ENABLE_ZRTP],[test "x$enable_zrtp" != "xno"])
 
 AM_CONDITIONAL([WANT_DEBUG],[test "${enable_debug}" = "yes"])
 
-enable_core_odbc_support="no"
 AC_ARG_ENABLE(core-odbc-support,     
-	[AS_HELP_STRING([--enable-core-odbc-support], [Compile with ODBC Support])],,[enable_core_odbc_support="no"])
+	[AS_HELP_STRING([--enable-core-odbc-support], [Compile with ODBC Support])],[enable_core_odbc_support="yes"],[enable_core_odbc_support="no"])
 if test "x$enable_core_odbc_support" != "xno"; then
   AX_LIB_ODBC
-  if test "x$ac_cv_found_odbc" = "xyes" ; then
+  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])
+  else
+    if test "x$enable_core_odbc_support" = "xyes"; then
+      AC_MSG_ERROR([no usable libodbc; please install unixodbc devel package or equivalent])
+    else
+      enable_core_odbc_support="no"
   fi
 fi