- Fixed Linux compiler errors and snd_mididevice.

-- errno.h is required for 'errno';
-- don't use str(n)casecmp and rely on ZDoom CMake handling;
-- add a missing parenthesis around a 'signed char' cast;
-- remove an unneeded GNU_SOURCE redefinition;
-- the non-MSVC side of snd_mididevice was not adapted to the new code, making wildmidi unavailable through the menu.

src/wildmidi/file_io.cpp

@@ -33,6 +33,8 @@
 **
 */
 
+#include <errno.h>
+
 #include "../files.h"
 #include "wm_error.h"
 #include "file_io.h"