Added missing m4 file ax_boost_chrono

Added missing m4 file ax_boost_chrono

Summary:
To avoid error while running ./configure command:
 ./configure: line 18976: AX_BOOST_CHRONO: command not found

Besides in my Fedora 23 the following error ocurred when not linked with -lboost_chrono running 'make check':
/usr/bin/ld: SynchronizedTest.o: undefined reference to symbol 'boost::chrono::steady_clock::now()''
/usr/lib64/libboost_chrono.so.1.58.0: error adding symbols: DSO missing from command line
Closes https://github.com/facebook/folly/pull/426

Differential Revision: D3457939

Pulled By: elliottneilclark

fbshipit-source-id: ec7a8d30d4812f197e2813e611cde0e33eab680a