Makefile: drop "-lpthread -lrt"

Both, -lpthread and -lrt seem to be relevant for api/ only. x86 already
adds these flags manually, when linking api/%. So we should be safe to
just drop it.

This makes it possible to easily cross-compile when e.g. the librt is not
available (or not linked to libc).

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>