vcl: make it easier to spot missing fonts (fontconfig backend)

Even better would be to show this on the UI, but that's more tricky:
it's not clear how to differentiate between the replacements which are
metric-compatible and the ones which are not.

Sample output:

PrintFontManager::Substitute: replacing missing font: 'Liberation Sansfoo' with 'Arial'

Change-Id: I96bf24f7e538fdfc163b9b6cab4b5a381bb4d760
Reviewed-on: https://gerrit.libreoffice.org/67342
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

vcl/unx/generic/fontmanager/fontconfig.cxx

@@ -1048,6 +1048,10 @@ void PrintFontManager::Substitute(FontSelectPattern &rPattern, OUString& rMissin
 
         FcFontSetDestroy( pSet );
     }
+
+    SAL_INFO("vcl.fonts", "PrintFontManager::Substitute: replacing missing font: '"
+                              << rPattern.maTargetName << "' with '" << rPattern.maSearchName
+                              << "'");
 }
 
 FontConfigFontOptions::~FontConfigFontOptions()