clone.2: Update feature test macro requirements

The requirements quietly changed changed in glibc 2.14

See also http://www.sourceware.org/bugzilla/show_bug.cgi?id=4749

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

man2/clone.2

@@ -47,9 +47,6 @@
 clone, __clone2 \- create a child process
 .SH SYNOPSIS
 .nf
-.BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
-.\" Actually _BSD_SOURCE || _SVID_SOURCE
-.\" FIXME See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
 .B #include <sched.h>
 
 .BI "int clone(int (*" "fn" ")(void *), void *" child_stack ,
@@ -57,6 +54,27 @@ clone, __clone2 \- create a child process
 .BI "          /* pid_t *" ptid ", struct user_desc *" tls \
 ", pid_t *" ctid " */ );"
 .fi
+.sp
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.sp
+.BR clone ():
+.ad l
+.RS 4
+.PD 0
+.TP 4
+Since glibc 2.14:
+_GNU_SOURCE
+.TP 4
+.\" FIXME See http://sources.redhat.com/bugzilla/show_bug.cgi?id=4749
+Before glibc 2.14:
+_BSD_SOURCE || _SVID_SOURCE
+    /* _GNU_SOURCE also suffices */
+.PD
+.RE
+.ad b
 .SH DESCRIPTION
 .BR clone ()
 creates a new process, in a manner similar to