Allow loading system OpenAL library on macOS again

In May 2017 the library loading was limited to *.dylib but the macOS
system OpenAL framework does not have dylib extension. So allow loading
files from /System/Library/Frameworks/ as libraries even without dylib
extension.

This is checked in Sys_DllExtension() so that QVM filesystem access will
not allow writing files to /System/Library/Frameworks/ even if homepath
is changed to include it. (Admittedly it doesn't fit the function name
but fits the function description and current usage.)

code/sys/sys_unix.c

@@ -928,6 +928,14 @@ qboolean Sys_DllExtension( const char *name ) {
 		return qtrue;
 	}
 
+#ifdef __APPLE__
+	// Allow system frameworks without dylib extensions
+	// i.e., /System/Library/Frameworks/OpenAL.framework/OpenAL
+	if ( strncmp( name, "/System/Library/Frameworks/", 27 ) == 0 ) {
+		return qtrue;
+	}
+#endif
+
 	// Check for format of filename.so.1.2.3
 	p = strstr( name, DLL_EXT "." );