Replace thread config with automatic config using ax_pthread.m4.

The autotools library macro (AX_PTHREAD) is now used to detect if
pthreads is present and multi-threaded linking in gold is automatically
enabled if it is found. This enables multi-threaded gold on platforms
where pthreads is enabled via other methods than just -lpthread
(e.g., MinGW).

2018-06-19  Joshua Watt  <jpewhacker@gmail.com>
	    Cary Coutant  <ccoutant@gmail.com>

gold/
	* configure.ac: Replace manual thread configuration with AX_PTHREAD.
	Add --enable-threads=auto.
	* Makefile.am (THREADFLAGS, THREADLIBS): New defines.
	(AM_CFLAGS, AM_CXXFLAGS): Add $(THREADFLAGS.
	(THREADSLIB): Remove; change all references to THREADLIBS.
	* Makefile.in: Regenerate.
	* aclocal.m4: Regenerate.
	* config.in: Regenerate.
	* configure: Regenerate.
	* testsuite/Makefile.am (THREADSLIB): Replace with...
	(THREADFLAGS, THREADLIBS): ... these.
	(LDADD): Remove; add as individual defines for...
	(object_unittest, binary_unittest, leb128_unittest)
	(overflow_unittest): ... these tests.
	(tls_test, tls_pic_test, tls_pie_test, tls_pie_pic_test)
	(tls_shared_test, tls_shared_ie_test, tls_shared_gd_to_ie_test)
	(tls_shared_gnu2_gd_to_ie_test, tls_shared_gnu2_test_LDFLAGS)
	(tls_shared_nonpic_test_LDFLAGS): Add $(THREADFLAGS) and ($THREADLIBS).
	* testsuite/Makefile.in: Regenerate.