Audio: work around issue with different typedef for ALbyte on OSX.

In OpenAL-Soft (Linux, Windows) `ALbyte` is defined as `signed char`,
while it is defined as `char` on OSX. Don't know why.

src/Audio/Audio.cpp

@@ -31,7 +31,12 @@ namespace Magnum { namespace Audio {
 
 /* Verify types */
 static_assert(std::is_same<ALubyte, UnsignedByte>::value, "ALubyte is not the same as UnsignedByte");
+/** @todo Why `ALbyte` is defined as `char` and not `signed char` on OSX? */
+#ifndef __APPLE__
 static_assert(std::is_same<ALbyte, Byte>::value, "ALbyte is not the same as Byte");
+#else
+static_assert(std::is_signed<ALbyte>::value && sizeof(ALbyte) == 1, "ALbyte does not have the same characteristics as Byte");
+#endif
 static_assert(std::is_same<ALushort, UnsignedShort>::value, "ALushort is not the same as UnsignedShort");
 static_assert(std::is_same<ALshort, Short>::value, "ALshort is not the same as Short");
 static_assert(std::is_same<ALuint, UnsignedInt>::value, "ALuint is not the same as UnsignedInt");