re PR target/41810 (Cannot build gcc: gthr-default.h:466: error: '__mutex' was not declared in this scope)

	PR target/41810
	* gthr-solaris.h (__gthread_mutex_destroy): Remove UNUSED.
	[SUPPORTS_WEAK && GTHREAD_USE_WEAK] (__gthread_active_p): Use
	__extension__ to allow cast from function pointer to object
	pointer in C++.
	* doc/install.texi (--enable-threads): Clarify use of Solaris
	threads.

From-SVN: r154708