Move feature defines to C files

color_math.c

@@ -1,3 +1,5 @@
+#define _USE_MATH_DEFINES
+#define _XOPEN_SOURCE 700
 #define _POSIX_C_SOURCE 200809L
 #include <math.h>
 #include <errno.h>

main.c

@@ -1,3 +1,5 @@
+#define _DEFAULT_SOURCE
+#define _XOPEN_SOURCE 700
 #include <assert.h>
 #include <errno.h>
 #include <fcntl.h>

meson.build

@@ -26,8 +26,6 @@ add_project_arguments(
 		'-Wno-unused-command-line-argument',
 		'-Wvla',
 		'-Wl,--exclude-libs=ALL',
-		'-D_USE_MATH_DEFINES',
-		'-D_XOPEN_SOURCE=700',
 		'-DWLSUNSET_VERSION="@0@"'.format(meson.project_version()),
 	],
 	language: 'c',

str_vec.c

@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE 700
 #include <stdlib.h>
 #include <string.h>