Fix libxslt format warnings

* config.h.in: Let autoconf have its way with the file.
* configure.in: Replace "-Wformat" with
"-Wformat=2 -Wmissing-format-attribute".  Add
"-Wformat=2 -Wmissing-format-attribute" to default CFLAGS.
* libxslt/transform.c:
(xsltDocumentElem): Fix missing placeholder in format string.
Make "unsupported method" format strings more consistent by
putting method name in parenthesis.
(xsltApplyStylesheetInternal): Ditto.
* libxslt/xsltconfig.h.in:
(LIBXSLT_ATTR_FORMAT): Add macro to decorate functions that
have format string arguments.
* libxslt/xsltutils.c:
(xsltGenericErrorDefaultFunc): Add macro to decorate functions
that have format string arguments.
(xsltGenericDebugDefaultFunc): Ditto.
* libxslt/xsltutils.h:
(xsltTransformError): Add LIBXSLT_ATTR_FORMAT macro.
* xsltproc/xsltproc.c:
(endTimer): Add predeclaration with LIBXSLT_ATTR_FORMAT macro.

Fixes bug #762006.

https://bugzilla.gnome.org/show_bug.cgi?id=762006