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: Enable compiler warnings to help add 'noreturn' attribute

This attributes helps the compiler to produce better binary code.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WINGs: Added attribute 'noreturn' to public function 'WMScreenMainLoop'

As suggested by GCC, this function is a good candidate. There is a little
constraint on how to do it however, as it is part of the public API so we
have to do it in a portable fashion (We can't rely on our "config.h" when
the file will have been installed).

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
util: Removed unused procedure that caused a compilation warning

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
util: Added noreturn attribute to appropriate functions

clang is a bit more strict on the attribute usage, so let's comply.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WPrefs: Added noreturn attribute to appropriate functions

clang is a bit more strict on the attribute usage, so let's comply.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
wrlib: Added noreturn attribute to appropriate functions

clang is a bit more strict on the attribute usage, so let's comply.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
wmaker: Do not call XShape function if the server did not say it supports it

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
WINGs: Do not create variables for trivial things

In the present case, it just led to a compiler warning on unused variable
when the XShape extension is disabled, but in general case it just adds
complexity by duplicating things; this should be reserved for complex
cases.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>
Remove some unnecessary code related to XShape usage

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