wrlib: Moved configure's detection of XPM support 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;
 - 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);
 - makes uses of shell functions to keep generated configure smaller by
sharing reusable stuff;
 - uses an automake conditional to avoid compiling the file is support is
not enabled

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

wrlib/xpm.c

@@ -1,4 +1,4 @@
-/* xpm.c - load XPM image from file
+/* xpm.c - load XPM image from file using libXpm
  *
  * Raster graphics library
  *
@@ -22,8 +22,6 @@
 
 #include <config.h>
 
-#ifdef USE_XPM
-
 #include <X11/Xlib.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -271,5 +269,3 @@ RImage *RLoadXPM(RContext * context, const char *file)
 	XpmFreeXpmImage(&xpm);
 	return image;
 }
-
-#endif				/* USE_XPM */