Make implicit_segmentation-related code an experiment.

This way, the code is not compiled in by default, thus decreasing
overall binary size.

Change-Id: I85cac8f5a22a51a7d99c820ef6d6ed179d4106a0
diff --git a/configure b/configure
index 77ae9de..c84136c 100755
--- a/configure
+++ b/configure
@@ -224,6 +224,7 @@
     switchable_interp
     newbestrefmv
     new_mvref
+    implicit_segmentation
 "
 CONFIG_LIST="
     external_build
diff --git a/vp8/vp8_common.mk b/vp8/vp8_common.mk
index c2eeff1..ed1923a 100644
--- a/vp8/vp8_common.mk
+++ b/vp8/vp8_common.mk
@@ -78,7 +78,7 @@
 VP8_COMMON_SRCS-yes += common/swapyv12buffer.c
 VP8_COMMON_SRCS-$(CONFIG_POSTPROC_VISUALIZER) += common/textblit.c
 VP8_COMMON_SRCS-yes += common/treecoder.c
-VP8_COMMON_SRCS-yes += common/implicit_segmentation.c
+VP8_COMMON_SRCS-$(CONFIG_IMPLICIT_SEGMENTATION) += common/implicit_segmentation.c
 
 VP8_COMMON_SRCS-$(ARCH_X86)$(ARCH_X86_64) += common/x86/idct_x86.h
 VP8_COMMON_SRCS-$(ARCH_X86)$(ARCH_X86_64) += common/x86/subpixel_x86.h