feature_test_macros.7: Add a summary of some FTM key points

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

man7/feature_test_macros.7

@@ -122,10 +122,70 @@ This format is employed in cases where only a single
 feature test macro can be used to expose the function
 declaration, and that macro is not defined by default.
 .SS Feature test macros understood by glibc
-The following paragraphs explain how feature test macros are handled
+The paragraphs below explain how feature test macros are handled
 in Linux glibc 2.\fIx\fP,
 .I x
 > 0.
+
+First, though a summary of a few details for the impatient:
+.IP * 3
+The macros that you most likely need to use in modern source code are
+.BR _POSIX_C_SOURCE
+(for definitions from various versions of POSIX.1),
+.BR _XOPEN_SOURCE
+(fof definitions from various versions of SUS),
+.BR _GNU_SOURCE
+(for GNU and/or Linux specific stuff), and
+.BR _DEFAULT_SOURCE
+(to get definitions that would normally be provided by default).
+.IP *
+Defining
+.BR _XOPEN_SOURCE
+with a value of 600 or greater produces the same effects as defining
+.BR _POSIX_C_SOURCE
+with a value of 200112L or greater.
+Where one sees
+.RS
+.nf
+
+    _POSIX_C_SOURCE >= 200112L
+
+.fi
+.RE
+.IP
+in the feature test macro requirements in the SYNOPSIS of a man page,
+it is implicit that the following has the same effect:
+.RS
+.nf
+
+    _XOPEN_SOURCE >= 600
+
+.fi
+.RE
+.IP *
+Defining
+.BR _XOPEN_SOURCE
+with a value of 700 or greater produces the same effects as defining
+.BR _POSIX_C_SOURCE
+with a value of 200809L or greater.
+Where one sees
+.RS
+.nf
+
+    _POSIX_C_SOURCE >= 200809L
+
+.fi
+.RE
+.IP
+in the feature test macro requirements in the SYNOPSIS of a man page,
+it is implicit that the following has the same effect:
+.RS
+.nf
+
+    _XOPEN_SOURCE >= 700
+
+.fi
+.RE
 .\" The details in glibc 2.0 are simpler, but combining a
 .\" a description of them with the details in later glibc versions
 .\" would make for a complicated description.