Use the correct definition to obtain snprintf(3)

At some point snprintf(3) prototype became guarded off in stdio.h.
Defining _C99_SOURCE makes it visible.
On legacy Darwin, such a guard was not present so the definition
has no impact on the build, but on "modern" macos it has the desired
effect.
Build tested on OS X 10.4 and macos 10.15.

build.sh

@@ -79,7 +79,7 @@ MSYS_NT*|MINGW*) # MSYS2 on Windows
 	fi
 	;;
 Darwin) # macOS
-	CFLAGS="${CFLAGS} -Wno-typedef-redefinition"
+	CFLAGS="${CFLAGS} -Wno-typedef-redefinition -D_C99_SOURCE"
 	UXNEMU_LDFLAGS="$(brew --prefix)/lib/libSDL2.a $(sdl2-config --cflags --static-libs | sed -e 's/-lSDL2 //')"
 	;;
 Linux|*)

src/devices/file.c

@@ -26,10 +26,6 @@
 #define PATH_MAX 4096
 #endif
 
-#ifdef __APPLE__
-int snprintf(char *, unsigned long, const char *, ...);
-#endif
-
 #include "../uxn.h"
 #include "file.h"