oox: add an environment variable to bypass the SmartArt warning

That dialog is counter-productive if you develop that feature itself.

Change-Id: Ia635a2ff0ecc29a0d11be5c621a7c2a76469f9ba
Reviewed-on: https://gerrit.libreoffice.org/61880Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins

oox/README.vars

+Environment variables in oox:
+
+- ppt:
+
+  - OOX_NO_SMARTART_WARNING - don't warn on SmartArts missing their DrawingML fallback

oox/source/ppt/pptimport.cxx

@@ -138,7 +138,8 @@ bool PowerPointImport::importDocument()
 
     bool bRet = importFragment(xPresentationFragmentHandler);
 
-    if (mbMissingExtDrawing)
+    static bool bNoSmartartWarning = getenv("OOX_NO_SMARTART_WARNING");
+    if (!bNoSmartartWarning && mbMissingExtDrawing)
     {
         // Construct a warning message.
         INetURLObject aURL(getFileUrl());