adjust sys/wait.h not to produde errors in strict ISO C feature profile

siginfo_t is not available from signal.h when the strict ISO C feature
profile (e.g. passing -std=c99 to gcc without defining any other
feature test macros) is used, but the type is needed to declare
waitid. using sys/wait.h (or any POSIX headers) in strict ISO C mode
is an application bug, but in the interest of compatibility, it's best
to avoid producing gratuitous errors. the simplest fix I could find is
suppressing the declaration of waitid (and also signal.h inclusion,
since it's not needed for anything else) in this case, while still
exposing everything else in sys/wait.h

include/sys/wait.h

@@ -6,8 +6,6 @@ extern "C" {
 
 #include <features.h>
 
-#include <signal.h>
-
 #define __NEED_pid_t
 #define __NEED_id_t
 #include <bits/alltypes.h>
@@ -19,9 +17,15 @@ typedef enum {
 } idtype_t;
 
 pid_t wait (int *);
-int waitid (idtype_t, id_t, siginfo_t *, int);
 pid_t waitpid (pid_t, int *, int );
 
+#if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
+ || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
+ || defined(_BSD_SOURCE)
+#include <signal.h>
+int waitid (idtype_t, id_t, siginfo_t *, int);
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 #include <sys/resource.h>
 pid_t wait3 (int *, int, struct rusage *);