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/Makefile.am

@@ -35,7 +35,6 @@ libwraster_la_SOURCES = 	\
 	rotate.c	\
 	convolve.c	\
 	nxpm.c		\
-	xpm.c 		\
 	xutil.c		\
 	ppm.c
 
@@ -55,6 +54,10 @@ if USE_TIFF
 libwraster_la_SOURCES += tiff.c
 endif
 
+if USE_XPM
+libwraster_la_SOURCES += xpm.c
+endif
+
 LTCOMPILE2=`echo $(LTCOMPILE) | sed -e s/-fomit-frame-pointer//`
 COMPILE2=`echo $(COMPILE) | sed -e s/-fomit-frame-pointer//`