Clean up use of Boost and PThread libraries.

Clean up use of Boost and PThread libraries.

Summary:
- Use AM_LDFLAGS rather than AM_CXXFLAGS to link against Boost.
- Link against $(BOOST_REGEX_LIB) in addition to other Boost libs.
- Add $(BOOST_LDFLAGS) to AM_LDFLAGS so that we get the library path
that we configured Boost to use.
- Add -lpthread to AM_LDFLAGS rather than ad-hoc '-lpthread' additions
to various library builds.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: oyamauchi@fb.com

FB internal diff: D998504