Require pkg-config 0.26 or later.

Earlier versions are buggy, as pkg-config --libs can output
nothing, which causes xrandr link failures.
* configure.ac (PKG_PROG_PKG_CONFIG): Bump from 0.9.0 to 0.26.

Fixes: debbugs:17438

ChangeLog

+2014-05-11  Paul Eggert  <eggert@cs.ucla.edu>
+
+	Require pkg-config 0.26 or later (Bug#17438).
+	Earlier versions are buggy, as pkg-config --libs can output
+	nothing, which causes xrandr link failures.
+	* configure.ac (PKG_PROG_PKG_CONFIG): Bump from 0.9.0 to 0.26.
+
 2014-05-07  Glenn Morris  <rgm@gnu.org>
 
 	* autogen.sh: Check for failing aclocal.

configure.ac

@@ -1349,7 +1349,7 @@ searched by pkg-config])
 pre_PKG_CONFIG_CFLAGS=$CFLAGS
 pre_PKG_CONFIG_LIBS=$LIBS
 
-PKG_PROG_PKG_CONFIG(0.9.0)
+PKG_PROG_PKG_CONFIG(0.26)
 
 dnl EMACS_CHECK_MODULES(GSTUFF, gtk+-2.0 >= 1.3 glib = 1.3.4)
 dnl acts like PKG_CHECK_MODULES(GSTUFF, gtk+-2.0 >= 1.3 glib = 1.3.4,