c89 compliant? (#237)

* Replace C++ comments into  C comments

* Move INLINE out of Makefile.common, move to fceu-types.h

An INLINE define is still existing in Makefile.common for msvc (dont
have a way to to test that). Moved the rest into fceu-types.h using the
one already existing from emu2413.h.

* Increase maximum volume for apu channels

src/boards/69.c

@@ -97,7 +97,7 @@ static DECLFW(M69Write1) {
 	}
 }
 
-// SUNSOFT-5/FME-7 Sound
+/* SUNSOFT-5/FME-7 Sound */
 
 static void AYSound(int Count);
 static void AYSoundHQ(void);