Only declare __sigpause in installed signal.h when necessary.

Only declare __sigpause in installed signal.h when necessary.

This patch makes the installed signal.h declare __sigpause only when
necessary (when a macro definition of sigpause makes use of
__sigpause), rather than unconditionally.  This fixes false positives
in the linknamespace tests by making it visible to those tests that no
use of ISO C functionality will actually bring in the definition of
__sigpause and so bring in the other symbols defined in the same
object.  There is no bug filed in Bugzilla because this is fixing
false positives rather than any user-visible bug.

Tested for x86_64 (testsuite, and that installed stripped shared
libraries are unchanged by this patch).

	* signal/signal.h (__sigpause): Only declare if [__USE_XOPEN &&
	!__GNUC__].
	* include/signal.h (__sigpause): Move declaration above call to
	libc_hidden_proto.
	* conform/Makefile (test-xfail-ISO/signal.h/linknamespace): Remove
	variable.
	(test-xfail-ISO99/signal.h/linknamespace): Likewise.
	(test-xfail-ISO11/signal.h/linknamespace): Likewise.