Enhance --enable-tunables to select tunables frontend at build time

At the GNU Tools Cauldron 2016, the state of the current tunables
patchset was considered OK with the addition of a way to select the
frontend to be used for the tunables.  That is, to avoid being locked
in to one type of frontend initially, it should be possible to build
tunables with a different frontend with something as simple as a
configure switch.

To that effect, this patch enhances the --enable-tunables option to
accept more values than just 'yes' or 'no'.  The current frontend (and
default when enable-tunables is 'yes') is called 'valstring', to
select the frontend where a single environment variable is set to a
colon-separated value string.  More such frontends can be added in
future.

	* Makeconfig (have-tunables): Check for non-negative instead
	of positive.
	* configure.ac: Add 'valstring' as a valid value for
	--enable-tunables.
	* configure: Regenerate.
	* elf/Makefile (have-tunables): Check for non-negative instead
	of positive.
	(CPPFLAGS-dl-tunables.c): Define TUNABLES_FRONTEND for
	dl-tunables.c.
	* elf/dl-tunables.c (GLIBC_TUNABLES): Define only when
	TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring.
	(tunables_strdup): Likewise.
	(disable_tunables): Likewise.
	(parse_tunables): Likewise.
	(__tunables_init): Process GLIBC_TUNABLES envvar only when.
	TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring.
	* elf/dl-tunables.h (TUNABLES_FRONTEND_valstring): New macro.
	(TUNABLES_FRONTEND_yes): New macro, define as
	TUNABLES_FRONTEND_valstring by default.
	* manual/install.texi: Document new acceptable values for
	--enable-tunables.
	* INSTALL: Regenerate.

1 files changed, 16 insertions, 1 deletions

diff --git a/manual/install.texi b/manual/install.texi
index d412962..2657da1 100644
--- a/manual/install.texi
+++ b/manual/install.texi
@@ -203,7 +203,22 @@ Use this option to disable the vector math library.
 @item --enable-tunables
 Tunables support allows additional library parameters to be customized at
 runtime.  This is an experimental feature and affects startup time and is thus
-disabled by default.
+disabled by default.  This option can take the following values:
+
+@table @code
+@item no
+This is the default if the option is not passed to configure. This disables
+tunables.
+
+@item yes
+This is the default if the option is passed to configure. This enables tunables
+and selects the default frontend (currently @samp{valstring}).
+
+@item valstring
+This enables tunables and selects the @samp{valstring} frontend for tunables.
+This frontend allows users to specify tunables as a colon-separated list in a
+single environment variable @env{GLIBC_TUNABLES}.
+@end table
 
 @item --build=@var{build-system}
 @itemx --host=@var{host-system}