split quant_levels.c into decoder and encoder version

-> split libraries further into decoder / encoder
-> add libwebpdecoder.a in Makefile.unix
-> make dwebp link against libwebpdecoder.a in Makefile.unix

also: in makefile.unix, pass EXTRA_FLAGS to LDFLAGS too
(otherwise, -m32 wouldn't work, e.g.)

Change-Id: Ief3da02a729dd86bbaf949ed048836716941657f

src/utils/quant_levels.h

@@ -27,11 +27,6 @@ extern "C" {
 int QuantizeLevels(uint8_t* const data, int width, int height, int num_levels,
                    uint64_t* const sse);
 
-// Apply post-processing to input 'data' of size 'width'x'height' assuming
-// that the source was quantized to a reduced number of levels.
-// Returns false in case of error (data is NULL, invalid parameters, ...).
-int DequantizeLevels(uint8_t* const data, int width, int height);
-
 #if defined(__cplusplus) || defined(c_plusplus)
 }    // extern "C"
 #endif