Use protected visibility for exported API functions

Protected visibility is like default visibility, in that functions will be
"exported" from the library. However, it also guarantees that references to the
functions from within the library will be to the library's version, even if the
symbols are overriden by the application.

include/AL/al.h

@@ -14,7 +14,7 @@ extern "C" {
  #endif
 #else
  #if defined(AL_BUILD_LIBRARY) && defined(HAVE_GCC_VISIBILITY)
-  #define AL_API __attribute__((visibility("default")))
+  #define AL_API __attribute__((visibility("protected")))
  #else
   #define AL_API extern
  #endif

include/AL/alc.h

@@ -14,7 +14,7 @@ extern "C" {
  #endif
 #else
  #if defined(AL_BUILD_LIBRARY) && defined(HAVE_GCC_VISIBILITY)
-  #define ALC_API __attribute__((visibility("default")))
+  #define ALC_API __attribute__((visibility("protected")))
  #else
   #define ALC_API extern
  #endif