vsnprintf-gnu: New module.

* m4/vsnprintf-posix.m4 (gl_FUNC_VSNPRINTF_IS_POSIX): New macro,
extracted from gl_FUNC_VSNPRINTF_POSIX.
(gl_FUNC_VSNPRINTF_POSIX): Require it. Invoke
gl_PREREQ_VASNPRINTF_WITH_POSIX_EXTRAS.
* m4/vsnprintf-gnu.m4: New file, based on m4/vsnprintf-posix.m4.
* modules/vsnprintf-gnu: New file, based on modules/vasnprintf-gnu.
* doc/posix-functions/vsnprintf.texi: Mention the new module.

modules/vsnprintf-gnu

@@ -0,0 +1,29 @@
+Description:
+POSIX and GNU compatible vsnprintf() function: print formatted output to
+a fixed length string
+
+Comment:
+This module should not be used as a dependency from a test module,
+otherwise when this module occurs as a tests-related module, it will
+have side effects on the compilation of the 'vasnprintf' module, if
+that module occurs among the main modules in lib/.
+
+Files:
+m4/vsnprintf-gnu.m4
+
+Depends-on:
+vsnprintf-posix
+
+configure.ac:
+gl_FUNC_VSNPRINTF_GNU
+
+Makefile.am:
+
+Include:
+<stdio.h>
+
+License:
+LGPL
+
+Maintainer:
+all