Build a threadsafe libjs so that JavaScript::SpiderMonkey can be threadsafe

Build a threadsafe libjs so that JavaScript::SpiderMonkey can be threadsafe

Thomas Berezansky <tsbere@mvlc.org> reported problems building
JavaScript::SpiderMonkey on 64-bit Ubuntu and offered some
patches to build it without -JS_THREADSAFE. The real problem, however,
was that libjs was not itself being built with the JS_THREADSAFE
option.

Enabling the JS_THREADSAFE option pulls in the requirement of
the nspr4-dev package on Ubuntu and Debian.

Tested on 64-bit Ubuntu Lucid and 64-bit Debian Lenny. 64-bit Fedora 13
worked previously because it uses the packaged version of libjs - which
was already built with the JS_THREADSAFE option.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@17052 dcc99617-32d9-48b4-a31d-7c20da2025e4