fix shared object build with -fvisibility=hidden

set WEBP_EXTERN to visibility=default
+ explicitly mark VP8GetCPUInfo as it's referenced within the examples

Change-Id: Ie3d2b15088e888f0b55203b205993eba75899d99

src/dsp/dsp.h

@@ -100,7 +100,7 @@ typedef enum {
 } CPUFeature;
 // returns true if the CPU supports the feature.
 typedef int (*VP8CPUInfo)(CPUFeature feature);
-extern VP8CPUInfo VP8GetCPUInfo;
+WEBP_EXTERN(VP8CPUInfo) VP8GetCPUInfo;
 
 //------------------------------------------------------------------------------
 // Encoding

src/webp/types.h

@@ -38,7 +38,11 @@ typedef long long int int64_t;
 #ifndef WEBP_EXTERN
 // This explicitly marks library functions and allows for changing the
 // signature for e.g., Windows DLL builds.
-#define WEBP_EXTERN(type) extern type
+# if defined(__GNUC__) && __GNUC__ >= 4
+#  define WEBP_EXTERN(type) extern __attribute__ ((visibility ("default"))) type
+# else
+#  define WEBP_EXTERN(type) extern type
+# endif  /* __GNUC__ >= 4 */
 #endif  /* WEBP_EXTERN */
 
 // Macro to check ABI compatibility (same major revision number)