Add configure check for std::thread

Add configure check for std::thread

This adds a configure check for std::thread.  This is needed because
std::thread is not available on some systems, like some versions of
mingw and DJGPP.

This also adds configury to make sure that a threaded gdb links
against the correct threading library (-lpthread or the like), and
passes the right flags (e.g., -pthread) to the compilations.

Note that this also links gdbserver against the thread library.  This
is not strictly necessary at this point in the series, but a later
patch will change gdbsupport to use pthread_sigmask, at which point
this will be needed.

gdb/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* acinclude.m4: Include ax_pthread.m4.
	* Makefile.in (PTHREAD_CFLAGS, PTHREAD_LIBS): New variables.
	(INTERNAL_CFLAGS_BASE): Use PTHREAD_CFLAGS.
	(CLIBS): Use PTHREAD_LIBS.
	(aclocal_m4_deps): Add ax_pthread.m4.
	* config.in, configure: Rebuild.
	* gdbsupport/common.m4 (GDB_AC_COMMON): Check for std::thread.

gdb/gdbserver/ChangeLog
2019-11-26  Tom Tromey  <tom@tromey.com>

	* Makefile.in (PTHREAD_CFLAGS, PTHREAD_LIBS): New variables.
	(INTERNAL_CFLAGS_BASE): Use PTHREAD_CFLAGS.
	(GDBSERVER_LIBS): Use PTHREAD_LIBS.
	* acinclude.m4: Include ax_pthread.m4.
	* config.in, configure: Rebuild.

Change-Id: I00ec55db6077f2615421a93461fc3be57e916aa0