gmp_4.2.1: prevent calls to mpn_add_nc() if HAVE_NATIVE_mpn_sub_nc is false
When building for aarch64 (ie relying only on generic C code rather
than asm) libgmp.so contains undefined references to __gmpn_add_nc
and __gmpn_sub_nc which causes attempts to link with -lgmp to fail:
| .../usr/lib/libgmp.so: undefined reference to `__gmpn_sub_nc'
| .../usr/lib/libgmp.so: undefined reference to `__gmpn_add_nc'
Solution based on a historical patch posted to the gmp mailing list:
https://gmplib.org/list-archives/gmp-discuss/2006-May/002344.html
Cherry-pick from meta-gplv2:
http://git.yoctoproject.org/cgit/cgit.cgi/meta-gplv2/commit/?id=
d8668018d5d795be2297f878fd871a27edf532bf
Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
