Add Boost.Atomic

[DrMcCoy]

Part of the changes I did on boost.m4 in https://github.com/xoreos/xoreos but never got around creating pull requests for. Sorry.

This adds a simple test for Boost.Atomic. However, due to the way the Boost library (or at least several versions of them) don't include all necessary headers, and those need to be included before the boost header, the CXX-PROLOGUE parameter handling in BOOST_FIND_LIB is changed a bit.

[pieterlexis]

after merge note: this breaks for unit test:

|....
| /* end confdefs.h.  */
| using boost::unit_test::test_suite;
|                test_suite* init_unit_test_suite(int argc, char ** argv)
|                { return NULL; }
| #include <boost/test/unit_test.hpp>
| int
| main ()
| {
| BOOST_CHECK(2 == 2);
|   ;
|   return 0;
| }
configure:19170: error: cannot compile a test that uses Boost unit_test_framework

Looking now