- Fix threading support, favor pthread
Since this port supports 5.x and later, we don't need to consider
the 4.x case (-lc_r). Moreover, gdc uses ld as linker when compiling
D source files, so PTHREAD_LIBS is not applicable here.
Submitted by: Jason DiCioccio <jd at ods.org>
Approved by: Masanori OZAWA <ozawa at ongs.co.jp> (maintainer)