automake conditional for visibility?

[Simon Josefsson]

Bruno, this part of the documentation for the "visibility" module
appears to assume that all public header files are generated from *.in
files via autoconf:

   Add a C macro definition, say @samp{-DBUILDING_LIBFOO}, to the CPPFLAGS
   for the compilation of the sources that make up the library.

   Define a macro specific to your library like this.
   @smallexample
   #if @@HAVE_VISIBILITY@@ && BUILDING_LIBFOO
   #define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
   #else
   #define LIBFOO_DLL_EXPORTED
   #endif
   @end smallexample
   This macro should be enabled in all public header files of your library.

How about adding a automake conditional?  That would allow adding the
-DBUILDING_LIBFOO only when visibility is supported, like this:

if HAVE_VISIBILITY
AM_CPPFLAGS += -DBUILDING_LIBFOO
endif

And have the public header file contain:

   #if BUILDING_LIBFOO
   #define LIBFOO_DLL_EXPORTED __attribute__((__visibility__("default")))
   #else
   #define LIBFOO_DLL_EXPORTED
   #endif

Btw, to encourage namespace clean header files, I would prefer
LIBFOO_BUILDING.

However, possibly the name of the CPP symbol used by the manual could be
modified to reflect that it also checks whether visibility is supported.
How about LIBFOO_ADD_VISIBILITY?

/Simon

diff --git a/m4/visibility.m4 b/m4/visibility.m4
index 70bca56..7b083b9 100644
--- a/m4/visibility.m4
+++ b/m4/visibility.m4
@@ -1,5 +1,5 @@
 # visibility.m4 serial 2 (gettext-0.18)
-dnl Copyright (C) 2005, 2008 Free Software Foundation, Inc.
+dnl Copyright (C) 2005, 2008, 2009 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
 dnl with or without modifications, as long as this notice is preserved.
@@ -49,4 +49,5 @@ AC_DEFUN([gl_VISIBILITY],
   AC_SUBST([HAVE_VISIBILITY])
   AC_DEFINE_UNQUOTED([HAVE_VISIBILITY], [$HAVE_VISIBILITY],
     [Define to 1 or 0, depending whether the compiler supports simple 
visibility declarations.])
+  AM_CONDITIONAL(HAVE_VISIBILITY, test "$HAVE_VISIBILITY" = "1")
 ])