Improve sysconf.h on Linux

I copied the constants and definitions from musl which we can presume is
correct. Relibc is missing some features which affects our sysconf as
well, such as constants in limits.h or a working resources.h.

include/bits/unistd.h

@@ -3,13 +3,6 @@
 
 #define _POSIX_VERSION 200809L
 #define _POSIX_REALTIME_SIGNALS 202405L
-
-#ifdef __cplusplus
-extern "C" {
-#endif
-
-#ifdef __cplusplus
-} // extern "C"
-#endif
+#define _XOPEN_VERSION 700L
 
 #endif