wrlib: Moved configure's detection of PNG 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

It includes a typo fixed by Amadeusz S-B³awiñski.-A

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

wrlib/Makefile.am

@@ -38,7 +38,6 @@ libwraster_la_SOURCES = 	\
 	xpm.c 		\
 	xutil.c		\
 	ppm.c		\
-	png.c 		\
 	tiff.c
 
 if USE_GIF
@@ -49,6 +48,10 @@ if USE_JPEG
 libwraster_la_SOURCES += jpeg.c
 endif
 
+if USE_PNG
+libwraster_la_SOURCES += png.c
+endif
+
 LTCOMPILE2=`echo $(LTCOMPILE) | sed -e s/-fomit-frame-pointer//`
 COMPILE2=`echo $(COMPILE) | sed -e s/-fomit-frame-pointer//`
 

wrlib/png.c

@@ -22,8 +22,6 @@
 
 #include <config.h>
 
-#ifdef USE_PNG
-
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -215,5 +213,3 @@ RImage *RLoadPNG(RContext *context, const char *file)
 	free(png_rows);
 	return image;
 }
-
-#endif				/* USE_PNG */