Always build the Mac preloader with '-mmacosx-version-min=10.7', which
works with all supported toolchains.
Fixes bug when building with Xcode 9 where preloader flags would be added
to EXTRACFLAGS and cause build failures on 32-bit.
Signed-off-by: Brendan Shanks <bshanks@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>