Configure: Moved configure's detection of XShm to a dedicated macro

The original check was not compliant with autoconf's syntax, did not have
a very good behaviour for user and was not easy to make evolve.

The new macro:
 - uses as much as possible autoconf macros for portability and code
consistency;
 - checks also for header usability with the compiler;
 - provides a consistent behaviour on yes/no/auto (if user explicitly
enables support, do not silently disable if not found; if library is found
but not the header, complain to let user install it or explicitly disable
support)

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

configure.ac

@@ -499,6 +499,16 @@ AC_ARG_ENABLE([shape],
     [enable_shape=auto])
 WM_XEXT_CHECK_XSHAPE
 
+dnl MIT-SHM support
+dnl ===============
+AC_ARG_ENABLE([shm],
+    [AS_HELP_STRING([--disable-shm], [disable usage of MIT-SHM extension])],
+    [AS_CASE(["$enableval"],
+        [yes|no], [],
+        [AC_MSG_ERROR([bad value $enableval for --enable-shm]) ]) ],
+    [enable_shm=auto])
+WM_XEXT_CHECK_XSHM
+
 dnl XINERAMA support
 dnl ================
 AC_ARG_ENABLE([xinerama],
@@ -587,28 +597,6 @@ AC_SUBST(XFTFLAGS)
 AC_SUBST(XFTLIBS)
 
 
-dnl MIT-SHM support
-dnl ===============
-shm=yes
-AC_ARG_ENABLE(shm, AS_HELP_STRING([--disable-shm], [disable usage of MIT-SHM extension]),
-		shm=$enableval, shm=yes)
-
-if test "$shm" = yes; then
-	AC_CHECK_LIB(Xext, XShmAttach, ok=yes, ok=no, $XLFLAGS $XLIBS)
-
-	if test "$ok" = yes; then
-	AC_CHECK_FUNC(shmget, ok=yes, ok=no)
-	fi
-
-	if test "$ok" = yes; then
-		if test "$added_xext" = no; then
-			XLIBS="-lXext $XLIBS"
-		fi
-                AC_DEFINE(USE_XSHM, 1, [define if X's shared memory extension is available (set by configure)])
-	fi
-fi
-
-
 dnl ==============================================
 dnl         Graphic Format Libraries
 dnl ==============================================