tzname, daylight, timezone: Update documentation

[Bruno Haible]

This patch updates the documentation regarding the three variables
  tzname
  daylight
  timezone
that are supposed to be set by tzset().

OpenBSD ≥ 5.8 supports these variables, but FreeBSD - to this day -
supports only 'tzname'.

On MSVC, these variables are not really missing; they just have a
different name, prefixed with underscore.


2024-06-06  Bruno Haible  <bruno@clisp.org>

        tzname, daylight, timezone: Update documentation.
        * doc/posix-functions/tzname.texi: Update info about native Windows.
        * doc/posix-functions/daylight.texi: Update info about OpenBSD, Cygwin,
        native Windows.
        * doc/posix-functions/timezone.texi: Update info about FreeBSD, OpenBSD,
        native Windows.

diff --git a/doc/posix-functions/daylight.texi 
b/doc/posix-functions/daylight.texi
index a6e5ce6c56..d5bfecedc1 100644
--- a/doc/posix-functions/daylight.texi
+++ b/doc/posix-functions/daylight.texi
@@ -14,10 +14,13 @@
 @itemize
 @item
 This variable is missing on some platforms:
-FreeBSD 14.0, OpenBSD 3.8.
+FreeBSD 14.0, OpenBSD 5.7.
+@item
+This variable is called @code{_daylight} on some platforms:
+mingw with @code{-DNO_OLDNAMES}, MSVC.
 @item
 The address of this variable is not a compile-time constant on some platforms:
-Cygwin, mingw.
+mingw, MSVC.
 @item
 Native Windows platforms (mingw, MSVC) support only a subset of time
 zones supported by GNU or specified by POSIX@.  @xref{tzset}.
diff --git a/doc/posix-functions/timezone.texi 
b/doc/posix-functions/timezone.texi
index b2647c46cd..5c040cab83 100644
--- a/doc/posix-functions/timezone.texi
+++ b/doc/posix-functions/timezone.texi
@@ -13,11 +13,15 @@
 Portability problems not fixed by Gnulib:
 @itemize
 @item
-This variable is missing on some platforms:
-mingw.
+This variable is missing (and instead, a function named @code{timezone} exists)
+on some platforms:
+FreeBSD 14.0, OpenBSD 5.7.
+@item
+This variable is called @code{_timezone} on some platforms:
+mingw with @code{-DNO_OLDNAMES}, MSVC.
 @item
 The address of this variable is not a compile-time constant on some platforms:
-mingw.
+mingw, MSVC.
 @item
 Native Windows platforms (mingw, MSVC) support only a subset of time
 zones supported by GNU or specified by POSIX@.  @xref{tzset}.
diff --git a/doc/posix-functions/tzname.texi b/doc/posix-functions/tzname.texi
index ef3f3d023a..d56ff5c924 100644
--- a/doc/posix-functions/tzname.texi
+++ b/doc/posix-functions/tzname.texi
@@ -13,11 +13,11 @@
 Portability problems not fixed by Gnulib:
 @itemize
 @item
-This variable is missing on some platforms:
-mingw.
+This variable is called @code{_tzname} on some platforms:
+mingw with @code{-DNO_OLDNAMES}, MSVC.
 @item
 The address of this variable is not a compile-time constant on some platforms:
-Cygwin, mingw.
+Cygwin, mingw, MSVC.
 @item
 Native Windows platforms (mingw, MSVC) support only a subset of time
 zones supported by GNU or specified by POSIX@.  @xref{tzset}.