Add "-pthread" to LDFLAGS.

This is required in order for GCC to pass the necessary flags
(e.g. "-lpthread") to the linker.

Change-Id: I5d62def362a95218cb30419aa21767e1ec32e4d2
Reviewed-on: https://code-review.googlesource.com/2181
Reviewed-by: Russ Cox <rsc@swtch.com>