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, 2 insertions, 1 deletions

diff --git a/configure b/configure
index d80d738fd1..eecd0ace74 100755
--- a/configure
+++ b/configure
@@ -1454,7 +1454,8 @@ Optional Features:
   --disable-build-nscd    disable building and installing the nscd daemon
   --disable-nscd          library functions will not contact the nscd daemon
   --enable-pt_chown       Enable building and installing pt_chown
-  --enable-tunables       Enable tunables support
+  --enable-tunables       Enable tunables support. Known values are 'yes',
+                          'no' and 'valstring'
   --enable-mathvec        Enable building and installing mathvec [default
                           depends on architecture]