Fix building non-GTK ports under Windows with Cairo >= 1.18

Cairo 1.18 broke using cairo_public redefinition hack used before to
avoid declaring Cairo functions as being dll-imported. Use the new way
to prevent this from happening but still define cairo_public when using
older versions.

Note that, unfortunately, we can't do it unconditionally because this
results in a macro redefinition warning with newer versions which is
impossible to disable using gcc.

As the new hack is even worse than the existing one, move it into its
own header to avoid duplicating it in both between src/common/cairo.cpp
and src/generic/graphicc.cpp.

Closes #26224.

include/wx/private/cairo.h

@@ -0,0 +1,29 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/cairo.h
+// Purpose:     Wrapper for including cairo.h from wx code
+// Author:      Vadim Zeitlin
+// Created:     2026-02-23
+// Copyright:   (c) 2026 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_CAIRO_H_
+#define _WX_PRIVATE_CAIRO_H_
+
+// We want to prevent cairo.h from using dllimport attribute for the functions
+// it declares as we're loading them dynamically, but this is trickier than it
+// should be because older versions of cairo.h respected cairo_public being
+// defined before including it, but this was broken in 1.18, so we need to
+// define an alternative symbol and avoid defining cairo_public as this would
+// result in a warning about its redefinition.
+#include <cairo-version.h>
+
+#if CAIRO_VERSION_MAJOR > 1 || CAIRO_VERSION_MINOR >= 18
+#define CAIRO_WIN32_STATIC_BUILD
+#else
+#define cairo_public
+#endif
+
+#include <cairo.h>
+
+#endif // _WX_PRIVATE_CAIRO_H_

src/common/cairo.cpp

@@ -14,11 +14,8 @@
 
 #if wxUSE_CAIRO && !defined(__WXGTK__)
 
-// keep cairo.h from defining dllimport as we're defining the symbols inside
-// the wx dll in order to load them dynamically.
-#define cairo_public
+#include "wx/private/cairo.h"
 
-#include <cairo.h>
 #include "wx/dynlib.h"
 
 #ifdef __WXMSW__

src/generic/graphicc.cpp

@@ -16,13 +16,8 @@
 
 #if wxUSE_CAIRO
 
-#ifndef __WXGTK__
-// keep cairo.h from defining dllimport as we're defining the symbols inside
-// the wx dll in order to load them dynamically.
-#define cairo_public
-#endif
+#include "wx/private/cairo.h"
 
-#include <cairo.h>
 #include <float.h>
 
 bool wxCairoInit();