add a --enable-experimental option to './configure'

This will enable USE_EXPERIMENTAL_FEATURES instead of having a header-level #define

Change-Id: I860b007f64220ddf92d0becd18f7d100c718f8d1

src/dec/Makefile.am

@@ -3,6 +3,7 @@ AM_CPPFLAGS = -I$(top_srcdir)/src
 libwebpdecode_la_SOURCES = bits.h vp8i.h yuv.h bits.c dsp.c frame.c \
                           quant.c tree.c vp8.c webp.c yuv.c idec.c alpha.c
 libwebpdecode_la_LDFLAGS = -version-info 0:0:0
+libwebpdecode_la_CPPFLAGS = $(USE_EXPERIMENTAL_CODE)
 libwebpdecodeinclude_HEADERS = ../webp/decode.h ../webp/decode_vp8.h ../webp/types.h
 libwebpdecodeincludedir = $(includedir)/webp
 

src/enc/Makefile.am

@@ -5,6 +5,7 @@ libwebpencode_la_SOURCES = analysis.c bit_writer.c bit_writer.h \
                           frame.c iterator.c picture.c quant.c  \
                           syntax.c tree.c vp8enci.h webpenc.c alpha.c
 libwebpencode_la_LDFLAGS = -version-info 0:0:0 -lm
+libwebpencode_la_CPPFLAGS = $(USE_EXPERIMENTAL_CODE)
 libwebpencodeinclude_HEADERS = ../webp/encode.h ../webp/types.h
 libwebpencodeincludedir = $(includedir)/webp
 

src/webp/types.h

@@ -12,10 +12,6 @@
 #ifndef WEBP_WEBP_TYPES_H_
 #define WEBP_WEBP_TYPES_H_
 
-// This is for experimentation only! Bitstreams generated will surely
-// be invalid, non-decodable ones! USE WITH CARE!
-// #define WEBP_EXPERIMENTAL_FEATURES   // activate alpha support, yuv444, etc.
-
 #ifndef _MSC_VER
 #include <inttypes.h>
 #ifdef ANSI