libpng: use GCC visibility to reduce shared library size

The change introduced the implementation for macro PNG_IMPEXP by using
GCC visibility in defaulting of "hidden" attribute.

Experiment on HTC Passion:
target file; out/target/product/passion/system/lib/libskia.so
(libpng was statically linked to libskia.so)

before: 1117888 bytes
after:  1113780 bytes

Code size reduction = 4108 bytes

Change-Id: I9b718a6a5710268dbb945feea3b64ca3fa393b94

2 files changed, 7 insertions, 2 deletions

diff --git a/Android.mk b/Android.mk
index 2a6eb43..d5c81d7 100644
--- a/Android.mk
+++ b/Android.mk
@@ -22,7 +22,8 @@ common_SRC_FILES := \
 	pngwtran.c \
 	pngwutil.c
 
-common_CFLAGS := ## -fomit-frame-pointer
+common_CFLAGS := \
+	-fvisibility=hidden ## -fomit-frame-pointer
 
 common_C_INCLUDES += \
 
diff --git a/pngconf.h b/pngconf.h
index defc16d..5760ba3 100644
--- a/pngconf.h
+++ b/pngconf.h
@@ -1505,7 +1505,11 @@ typedef z_stream FAR *  png_zstreamp;
 #  define PNGAPI
 #endif
 #ifndef PNG_IMPEXP
-#  define PNG_IMPEXP
+#  if ((__GNUC__-0) * 10 + __GNUC_MINOR__-0 >= 33)
+#    define PNG_IMPEXP  __attribute__((visibility ("default")))
+#  else
+#    define PNG_IMPEXP
+#  endif
 #endif
 
 #ifdef PNG_BUILDSYMS