meson: add DEFS to CFLAGS Adding a configuration flag as an include directory is wrong and errors on BSDs. ERROR: Include dir -DU_DISABLE_RENAMING=1 does not exist. Matches the autotools build now. Requires has_argument to fix non BSDs because of an annoying meson quirk. Signed-off-by: Rosen Penev <rosenp@gmail.com>