psol: support a binary distribution

We want people to be able to install ngx_pagespeed like any other nginx module.
If PSOL were a full-fledged library with a stable interface we could simply say
"install PSOL" which might be as easy as `apt-get install psol-dev`, but it's
not.  Instead we directly include a precompiled binary library along with its
headers.  By building the binaries on something sufficiently old (Centos 5.4)
they should work pretty much everywhere someone might think to run
ngx_pagespeed, but building from source is still a documented option if they
don't.

Headers and binaries are from trunk of r2338, as documented in the READMEs.