Globally define _POSIX_C_SOURCE in meson.build

Fix musl issue with time.h

meson.build

@@ -17,6 +17,7 @@ add_project_arguments(cc.get_supported_arguments([
 	'-Wno-missing-braces',
 	'-Wno-missing-field-initializers',
 	'-Wno-unused-parameter',
+	'-D_POSIX_C_SOURCE=200809L',
 ]), language: 'c')
 
 inc = include_directories('include')

src/screencast/screencast.c

@@ -1,5 +1,3 @@
-#define _POSIX_C_SOURCE 200809L
-
 #include "screencast.h"
 
 #include <errno.h>

src/screencast/wlr_screencast.c

@@ -1,5 +1,3 @@
-#define _POSIX_C_SOURCE 200809L
-
 #include "wlr_screencast.h"
 
 #include "wlr-screencopy-unstable-v1-client-protocol.h"