The standard Boost convention is for them to be called libboost_date_time and
libboost_system (with apropriate suffix then depending on type).

Did not touch the libboostthread library we build for Windows.

Add the ax_boost_thread.m4 file for completeness and possible use.

Document where the ax_boost*.m4 files come from.

Change-Id: Ib49bee71398d62c9760a1f8fd5c46be9f3400430