sigaction.2: Note feature test macro requirements for 'si_code' constants

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/sigaction.2

@@ -549,7 +549,35 @@ and have the ptrace event in the high byte:
 .nf
     (SIGTRAP | PTRACE_EVENT_foo << 8).
 .fi
-
+.PP
+For a
+.RB non- ptrace (2)
+event, the values that can appear in
+.I si_code
+are described in the remainder of this section.
+Since glibc 2.20,
+the definitions of most of these symbols are obtained from
+.I <signal.h>
+by defining feature test macros (before including
+.I any
+header file) as follows:
+.IP * 3
+.B _XOPEN_SOURCE
+with the value 500 or greater;
+.IP *
+.B _XOPEN_SOURCE
+and
+.BR _XOPEN_SOURCE_EXTENDED ;
+or
+.IP *
+.B _POSIX_C_SOURCE
+with the value 200809L or greater.
+.PP
+For the
+.B TRAP_*
+constants, the symbol definitions are provided only in the first two cases.
+Before glibc 2.20, no feature test macros were required to obtain these symbols.
+.PP
 For a regular signal, the following list shows the values which can be
 placed in
 .I si_code