configure: Moved configure's detection of XShape 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>

m4/wm_xext_check.m4

@@ -0,0 +1,39 @@
+# wm_xext_check.m4 - Macros to check for X extensions support libraries
+#
+# Copyright (c) 2013 Christophe CURIS
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+# WM_XEXT_CHECK_XSHAPE
+# --------------------
+#
+# Check for the X Shaped Window extension
+# The check depends on variable 'enable_xshape' being either:
+#   yes  - detect, fail if not found
+#   no   - do not detect, disable support
+#   auto - detect, disable if not found
+#
+# When found, append appropriate stuff in XLIBS, and append info to
+# the variable 'supported_xext'
+# When not found, append info to variable 'unsupported'
+AC_DEFUN_ONCE([WM_XEXT_CHECK_XSHAPE],
+[WM_LIB_CHECK([XShape], [-lXext], [XShapeSelectInput], [$XLIBS],
+    [wm_save_CFLAGS="$CFLAGS"
+     AS_IF([wm_fn_lib_try_compile "X11/extensions/shape.h" "Window win;" "XShapeSelectInput(NULL, win, 0)" ""],
+        [],
+        [AC_MSG_ERROR([found $CACHEVAR but cannot compile using XShape header])])
+     CFLAGS="$wm_save_CFLAGS"],
+    [supported_xext], [XLIBS], [enable_shape], [-])dnl
+]) dnl AC_DEFUN