cc/gcc: fix thread-less final gcc build
If threads are disabled in libc, we don't want to enable them in the
final compiler. Doing so pass the configure stage, but fails latter on
a missing <pthread.h>.
Moreover, we don't want to build libgomp if threads are disabled; its
configure script would fails anyway.
Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>