Remove custom pthread_once implementation on sh.
Add macros for diagnostic control, use for scanf %a tests.

In <https://sourceware.org/ml/libc-alpha/2014-11/msg00326.html>,
Roland requested internal macros for use of "#pragma GCC diagnostic".

This patch adds such macros and uses them to disable -Wformat warnings
for some code testing GNU scanf %as where GCC expects C99 scanf %a
(several other stdio tests currently use -Wno-format to disable
warnings).  Limitations in GCC's diagnostic pragmas require separate
macros before and after the code generating the warnings, rather than
a single macro taking that code as an argument.

The macros are named DIAG_*_NEEDS_COMMENT to emphasise to reviewers
the need for a comment accompanying any use of them (such comments may
however just appear once for several uses of the macros for the same
issue in the same file).  I put a GCC version in the arguments to
DIAG_IGNORE_NEEDS_COMMENT, as that seems something useful to grep for
when obsoleting support for an old GCC version and needing to decide
if warning-disabling code is still relevant.

These macros should be usable for replacing existing -Wno-* use in
makefiles (as also suggested by Roland), though I have no plans to
work on that (only on use of the macros in cases where warnings are
currently present that need disabling to use -Werror).

Tested for x86_64.

	* include/libc-internal.h (DIAG_PUSH_NEEDS_COMMENT): New macro.
	(DIAG_POP_NEEDS_COMMENT): Likewise.
	(_DIAG_STR1): Likewise.
	(_DIAG_STR): Likewise.
	(DIAG_IGNORE_NEEDS_COMMENT): Likewise.
	* stdio-common/bug21.c: Include <libc-internal.h>.
	(do_test): Disable -Wformat around call to sscanf.
	* stdio-common/scanf14.c: Include <libc-internal.h>.
	(main): Disable -Wformat around some calls to scanf functions.
Disable -Wdiv-by-zero for some tests in stdio-common/tst-unlockedio.c.

This patch uses diagnostic control pragmas to disable some
division-by-zero warnings in stdio-common/tst-unlockedio.c.  This is
another test where disabling warnings using diagnostic pragmas seems
appropriate (the warnings are not wildly inappropriate for what the
test deliberately does; the deliberate use of unusual arguments to the
macros being tested means that changing the arguments in a way that
avoids the warning would also unduly perturb what is being tested).

Tested for x86_64.

	* stdio-common/tst-unlockedio.c: Include <libc-internal.h>.
	(do_test): Disable -Wdiv-by-zero around some calls to
	fwrite_unlocked and fread_unlocked.
Disable -Wdeprecated-declarations for register_printf_function calls in tst-printfsz.c.

This patch uses diagnostic control pragmas to disable warnings in
stdio-common/tst-printfsz.c for use of the deprecated
register_printf_function.  Because this test is testing printf_size
and printf_size_info, and the latter has the interface expected for
register_printf_function instead of the newer
register_printf_specifier, it seems correct for this test to use the
deprecated interface (wrapping printf_size_info in some way to use
register_printf_specifier would seem an excessive change to what's
tested).

Tested for x86_64.

	* stdio-common/tst-printfsz.c: Include <libc-internal.h>.
	(main): Disable -Wdeprecated-declarations around calls to
	register_printf_function.
Use -Werror by default, add --disable-werror.

As discussed starting at
<https://sourceware.org/ml/libc-alpha/2014-11/msg00323.html>, this
patch makes the glibc build use -Werror by default to avoid
accidentally adding new warnings to the build.  The configure option
--disable-werror can be used to disable this.

-Wno-error=undef is temporarily used because the build isn't clean
regarding -Wundef warnings.  The idea is that once the remaining
-Wundef warnings have been cleaned up (in at least one configuration),
-Wno-error=undef will be removed.

I get a clean build and test on x86_64 (GCC 4.9 branch) with this
patch.  The expectation is that this may well break the build for some
other configurations, and people seeing such breakage should make
appropriate fixes to fix or suppress the warnings for their
configurations.  In some cases that may involve using pragmas as the
right fix (I think that will be right for the -Wno-inline issue for
MIPS I referred to in
<https://sourceware.org/ml/libc-alpha/2012-11/msg00798.html>, for
example), in some cases -Wno-error in sysdeps makefiles (__restore_rt
in MIPS sigaction, for example), in some cases substantive fixes for
the warnings.

Note that if, with a view to listing all the warnings then fixing them
all, you just look for "warning:" in output from building and testing
with --disable-werror, you'll see lots of warnings from the linker
about functions such as tmpnam.  Those warnings can be ignored - only
compiler warnings are relevant to -Werror, not linker warnings.

	* configure.ac (--disable-werror): New configure option.
	(enable_werror): New AC_SUBST.
	* configure: Regenerated.
	* config.make.in (enable-werror): New variable.
	* Makeconfig [$(enable-werror) = yes] (+gccwarn): Add -Werror
	-Wno-error=undef.
	(+gccwarn-c): Do not use -Werror=implicit-function-declaration.
	* manual/install.texi (Configuring and compiling): Document
	--disable-werror.
	* INSTALL: Regenerated.
	* debug/Makefile (CFLAGS-tst-chk1.c): Add -Wno-error.
	(CFLAGS-tst-chk2.c): Likewise.
	(CFLAGS-tst-chk3.c): Likewise.
	(CFLAGS-tst-chk4.cc): Likewise.
	(CFLAGS-tst-chk5.cc): Likewise.
	(CFLAGS-tst-chk6.cc): Likewise.
	(CFLAGS-tst-lfschk1.c): Likewise.
	(CFLAGS-tst-lfschk2.c): Likewise.
	(CFLAGS-tst-lfschk3.c): Likewise.
	(CFLAGS-tst-lfschk4.cc): Likewise.
	(CFLAGS-tst-lfschk5.cc): Likewise.
	(CFLAGS-tst-lfschk6.cc): Likewise.