re PR bootstrap/9330 ([alpha-osf]small alphaev56-dec-osf5.1 --enable-threads=posix problem)

	* config.gcc (alpha*-dec-osf[45]*): Enable POSIX thread support by
	default.

	* gthr-posix.c: New file.
	* gthr-posix.h: Define _REENTRANT if missing.
	Make _LIBOBJC #pragma weak visible with _LIBOBJC_WEAK.

	* config/alpha/t-osf4 (SHLIB_LINK): Hide dummy functions provided
	by gthr-posix.o.
	* config/alpha/t-osf-pthread: New file.

	* fixinc/inclhack.def (alpha_pthread): New fix.
	* fixinc/fixincl.x: Regenerate.
	* fixinc/tests/base/pthread.h [ALPHA_PTHREAD_CHECK]: New testcase.

	* doc/install.texi (alpha*-dec-osf*): Remove --enable-threads
	warning.
	Fixes PR bootstrap/9330.

From-SVN: r69998