compile dyncall with -fPIC on NetBSD
This fixes a build problem on NetBSD.
It's the same fix as in commit 5ed99d26b3c3ac2d65bc622afdbd987a547046f5 for FreeBSD.
OpenBSD does not need CFLAGS=-fPIC (was able to build parrot and perl6-p on OpenBSD 5.5).