mmap.2: Some 'flags' values require a feature test macro to be defined

Add text to NOTES noting that some MAP_* constants are
defined only if a suitable feature test macro is defined.
See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=542601

Reported-by: Török Edwin <edwintorok@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/mmap.2

@@ -37,7 +37,7 @@
 .\" 2007-07-10, mtk, Added an example program.
 .\" 2008-11-18, mtk, document MAP_STACK
 .\"
-.TH MMAP 2 2012-04-16 "Linux" "Linux Programmer's Manual"
+.TH MMAP 2 2013-02-13 "Linux" "Linux Programmer's Manual"
 .SH NAME
 mmap, munmap \- map or unmap files or devices into memory
 .SH SYNOPSIS
@@ -49,6 +49,8 @@ mmap, munmap \- map or unmap files or devices into memory
 .BI "           int " fd ", off_t " offset );
 .BI "int munmap(void *" addr ", size_t " length );
 .fi
+
+See NOTES for information on feature test macro requirements.
 .SH DESCRIPTION
 .BR mmap ()
 creates a new mapping in the virtual address space of
@@ -541,6 +543,36 @@ If the
 flag is specified, and
 .I addr
 is 0 (NULL), then the mapped address will be 0 (NULL).
+
+Certain
+.I flags
+constants are defined only if either
+.BR _BSD_SOURCE
+or
+.BR _SVID_SOURCE
+is defined.
+(Requiring
+.BR _GNU_SOURCE
+also suffices,
+and requiring that macro specifically would have been more logical,
+since these flags are all Linux specific.)
+The relevant flags are:
+.BR MAP_32BIT ,
+.BR MAP_ANONYMOUS
+(and the synonym
+.BR MAP_ANON ),
+.BR MAP_DENYWRITE ,
+.BR MAP_EXECUTABLE ,
+.BR MAP_FILE ,
+.BR MAP_GROWSDOWN ,
+.BR MAP_HUGETLB ,
+.BR MAP_LOCKED ,
+.BR MAP_NONBLOCK ,
+.BR MAP_NORESERVE ,
+.BR MAP_POPULATE ,
+and
+.BR MAP_STACK .
+
 .SH BUGS
 On Linux there are no guarantees like those suggested above under
 .BR MAP_NORESERVE .