RFC: trace-threads branch

[José Fonseca]

The "trace-threads" branch adds support to distinguish the thread
where each call is made:

  https://github.com/apitrace/apitrace/commits/trace-threads

One side effect is that pthread library is linked explicitly
(necessary because on some distros glibc pthread stubs don't include
the TLS-related entrypoints), which means that single threaded apps
will be slower as mutex calls won't be redirected to the pthreads
stubs. This doesn't affect DRI drivers, as it looks like libGL.so /
foo_dri.so already explicitly link against pthreads, hence, no change
there.

I'd like to merge this in 1-2 weeks time.  I'd particular like to know
if there are any regressions on android, as I've already tested on
other OSes.

Jose