Update ax_boost_base.m4 to serial 21

- Add aarch64 to the list of lib64 architectures
Update ax_boost_filesystem.m4 to serial 26

- Fixed cut-and-paste error.
- See http://savannah.gnu.org/patch/?7576 for further details.
- Use 'ls -r' rather than non-portable tool 'tac'.
- Simplify filepattern in ls expression so dylib (on OSX) are also detected.
Update ax_boost_program_options.m4 serial 22

- Fixed detection of dynamic libraries on Mac OS X (*.dylib)

See http://savannah.gnu.org/patch/?7719 for further details.
Update ax_boost_system.m4 to serial 17

- Use 'ls -r' rather than non-portable tool 'tac'. Simplify filepattern in ls expression so dylib (on OSX) are also detected.
Update ax_boost_thread.m4 to serial 27

- Use 'ls -r' in order to try newer version before older ones. Simplify
  ls pattern so also dylibs are detected.
Update ax_pthread.m4 to serial 21

- Add support for Clang

Further details can be found at http://savannah.gnu.org/patch/?8186
Merge pull request #3278 from laanwj/2013_11_boost_m4

Update boost m4 files
Merge pull request #7 from bitcoin/master

Update boost m4 files