Allow leaving the point size unspecified for ports using XLFD

Rather than defaulting to the normal font size, just leave the font size
unspecified as it might result in better font size being chosen.

src/unix/fontutil.cpp

@@ -795,8 +795,13 @@ wxFontEncoding wxNativeFontInfo::GetEncoding() const
 
 void wxNativeFontInfo::SetFractionalPointSize(float pointsize)
 {
-    SetXFontComponent(wxXLFD_POINTSIZE,
-                      wxString::Format("%d", wxRound(10*pointsize)));
+    wxString s;
+    if ( pointsize < 0 )
+        s = '*';
+    else
+        s.Printf("%d", wxRound(10*pointsize));
+
+    SetXFontComponent(wxXLFD_POINTSIZE, s);
 }
 
 void wxNativeFontInfo::SetStyle(wxFontStyle style)