Enable workaround for building on powerpc64.

Autoconf reports that HAVE_ASM_BSR is true (1). However, it's a false
positive.

I do not have enough knowledge of autoconf to fix this problem correctly,
so fix src/config.h in post-configure. Patches welcome.

A proper fix will most likely also fix the build on other tier-2 archs,
but I have not taken the time to test that yet.

Approved by: portmgr (tier-2 blanket)