-lm must come after the source file, versions of gcc insist this strict order.
On standard compliant systems, we no longer need malloc.h for malloc.
Use pkg-config(1) to get correct glib cflags and libs.
Fix compiler warning in threadring.c and k-nucleotide.c.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6198076