tests: Use -pthread in CFLAGS instead of -lpthread

-lpthread is not always a valid flag to pull pthread support, especially
on Android it will fail to link due to a missing libpthread.so. The more
generic way to build-in pthread support is to use the -pthread CFLAG, so
let's use it instead.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
[Emil Velikov: rebase on top of previous commit]
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>