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/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 */