utils.c: include <signal.h> for siginfo_t (#7517)

* utils.c: include <signal.h> for siginfo_t

POSIX says:

> The <signal.h> header shall define the siginfo_t type as a structure

So <sys/wait.h> is not enough to see the definition (not just a forward
declaration) of siginfo_t.

* NEWS entry

* Amend NEWS

* more robustly define _POSIX_C_SOURCE (h/t Hugh)

* tidy up NEWS

* -D_POSIX_C_SOURCE=200809L in gitlab CI job for regression test

* revert gitlab-ci change

---------

Co-authored-by: Michael Chirico <[email protected]>
Co-authored-by: Michael Chirico <[email protected]>

Author: aitap

NEWS.md

@@ -8,6 +8,8 @@
 
 1. Removed use of non-API `ATTRIB`, `SET_ATTRIB`, and `findVar` [#6180](https://github.com/Rdatatable/data.table/issues/6180). Thanks @aitap for the continued assiduous work here, and @MichaelChirico for the easy fix to replace `findVar` with `R_getVar`.
 
+7. Fixed compilation failure like "error: unknown type name 'siginfo_t'" in v1.18.0 in some strict environments, e.g., FreeBSD, where the header file declaring the POSIX function `waitid` does not transitively include the header file defining the `siginfo_t` type, [#7516](https://github.com/rdatatable/data.table/issues/7516). Thanks to @jszhao for the report and @aitap for the fix.
+
 ### Notes
 
 1. {data.table} now depends on R 3.5.0 (2018).

src/utils.c

@@ -1,5 +1,10 @@
 #ifndef _WIN32
-#  include <sys/wait.h>
+#  if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L
+#    undef _POSIX_C_SOURCE
+#    define _POSIX_C_SOURCE 200809L // required for POSIX (not standard C) features in is_direct_child e.g. 'siginfo_t'
+#  endif
+#  include <signal.h> // siginfo_t
+#  include <sys/wait.h> // waitid
 #endif
 
 #include "data.table.h"