Fix mod_spidermonkey build on FreeBSD, (Undefined symbol

Fix mod_spidermonkey build on FreeBSD, (Undefined symbol
PR_LocalTimeParameters).

Move nspr libs to AM_LIBADD to make sure they are _after_ libjs.so.1,
so missing PR_* symbols in libjs can be resolved.

Having all those nspr libs in AM_LDFLAGS will make them to end up before
libjs on the final linker commandline.

The wrong order caused the build-time linker to discard nspr,
since nothing had unresolved symbols belonging to it at that point.
(mod_spidermonkey does not use any PR_* functions/objects.)

Compile + load tested on FreeBSD-8.1.