configure: Enable compiler warnings to help add 'format' attribute

This attribute helps the compiler checking the type matching between
arguments and the specification in a printf-like format string, to
avoid invalid output. This attribute is optional, but some compilers
can suggest functions that could have it.

This patch adds the appropriate compiler flags if they are supported when
the source is being compiled with DEBUG enabled.

The patch also introduces a new macro WM_CFLAGS_CHECK_FIRST because in some
cases AX_CFLAGS_GCC_OPTION is not really efficient and in present case it
does not fits the job.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

configure.ac

@@ -129,6 +129,12 @@ AS_IF([test "x$debug" = "xyes"],
      dnl Having more than 1 prototype for a function makes code updates
      dnl more difficult, so try to avoid it
      AX_CFLAGS_GCC_OPTION([-Wredundant-decls])
+     dnl
+     dnl Proper attributes helps the compiler to produce better code
+     WM_CFLAGS_CHECK_FIRST([format attribute suggest],
+         [-Wsuggest-attribute=format  dnl new gcc syntax
+          -Wmissing-format-attribute  dnl old gcc syntax, clang
+         ])
 ], [dnl
      dnl When debug not enabled, we try to avoid some non-necessary
      dnl messages from the compiler