wrlib: Added support for webp image

This patch is adding support for google webp image format, if you
don't know it a quick recap is to say that according to their tests
they claim it is better than png and jpeg.

Follow the link below for some more details:
https://developers.google.com/speed/webp/

Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>

wrlib/Makefile.am

@@ -60,6 +60,10 @@ else
 libwraster_la_SOURCES += load_xpm_normalized.c
 endif
 
+if USE_WEBP
+libwraster_la_SOURCES += load_webp.c
+endif
+
 AM_CPPFLAGS = $(DFLAGS) @HEADER_SEARCH_PATH@
 
 libwraster_la_LIBADD = @LIBRARY_SEARCH_PATH@ @GFXLIBS@ @XLIBS@ @LIBXMU@ -lm