Use "internal" visibility by default

All functions callable by apps, directly or indirectly (eg, through function
pointers), are marked as "protected", and internal visibility can provide
benefits over hidden when we know functions won't be called by other modules.

CMakeLists.txt

@@ -133,7 +133,7 @@ ELSE()
             CHECK_C_SOURCE_COMPILES("int foo() __attribute__((visibility(\"default\")));
                                      int main() {return 0;}" HAVE_GCC_VISIBILITY)
             IF(HAVE_GCC_VISIBILITY)
-                ADD_DEFINITIONS(-fvisibility=hidden -DHAVE_GCC_VISIBILITY)
+                ADD_DEFINITIONS(-fvisibility=internal -DHAVE_GCC_VISIBILITY)
             ENDIF()
         ENDIF()
     ENDIF()