flex: fix configure cache issue with BR2_NEEDS_GETTEXT

Flex doesn't NEED gettext/libintl, but it's configure script checks for it,
so make sure those a built before flex, otherwise flex will populate
tgt-config.cache with invalid values, breaking the build of other packages
needing it (like libglib2).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>