Generate error message in case bibtex generation fails

In case bibtex fails no message is given. When perl is missing the OS just emits an message. With this patch doxygen logs a message (analogous to other calls to portable_system)

src/cite.cpp

@@ -211,8 +211,15 @@ void CiteDict::generatePage() const
 
   // 5. run bib2xhtml perl script on the generated file which will insert the
   //    bibliography in citelist.doc
-  portable_system("perl","\""+bib2xhtmlFile+"\" "+bibOutputFiles+" \""+
-                         citeListFile+"\"");
+  int exitCode;
+  portable_sysTimerStop();
+  if ((exitCode=portable_system("perl","\""+bib2xhtmlFile+"\" "+bibOutputFiles+" \""+
+                         citeListFile+"\"")) != 0)
+  {
+    err("Problems running bibtex. Verify that the command 'perl --version' works from the command line. Exit code: %d\n",
+        exitCode);
+  }
+  portable_sysTimerStop();
 
   QDir::setCurrent(oldDir);