Protect counter with mutex (regression)

I checked PR doxygen#11902 and found a small problem when `DOT_CLEANUP=YES` as I get the warnings:
```
.../aa.c:17: error: Problems running dot: exit code=2, command='dot.exe', arguments='".../doc/html/inline_dotgraph_4b689ea5cd4847e4f5ac9ff55d29fff0.dot" -Tcmapx -o ".../doc/html/dot_inline_dotgraph_4b689ea5cd4847e4f5ac9ff55d29fff0.map"'
Failed to open input file '.../doc/html/inline_mscgraph_7c6951245abdf3122b13597d42cd76cb.msc'
.../bb.c:15: error: Problems generating msc output (error=FILE ERROR). Look for typos in you msc file '.../doc/html/inline_mscgraph_7c6951245abdf3122b13597d42cd76cb.msc'
```

Delayed deletion of files until all code has been written.

Author: albert-github

src/docbookvisitor.cpp

@@ -365,7 +365,6 @@ DB_VIS_C
         if (!fileName.isEmpty())
         {
           writeDotFile(fileName, s, !exists);
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
         m_t << "</para>\n";
       }
@@ -381,7 +380,6 @@ DB_VIS_C
         if (!fileName.isEmpty())
         {
           writeMscFile(fileName,s,!exists);
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
         m_t << "</para>\n";
       }

src/doxygen.cpp

@@ -13403,6 +13403,7 @@ void generateOutput()
   }
 
   g_outputList->cleanup();
+  cleanupInlineGraph();
 
   msg("type lookup cache used {}/{} hits={} misses={}\n",
       Doxygen::typeLookupCache->size(),

src/htmldocvisitor.cpp

@@ -600,8 +600,6 @@ void HtmlDocVisitor::operator()(const DocVerbatim &s)
           writeDotFile(fileName,s.relPath(),s.context(),s.srcFile(),s.srcLine(),!exists);
           visitCaption(m_t, s);
           m_t << "</div>\n";
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
 
         forceStartParagraph(s);
@@ -622,8 +620,6 @@ void HtmlDocVisitor::operator()(const DocVerbatim &s)
           writeMscFile(fileName,s.relPath(),s.context(),s.srcFile(),s.srcLine(),!exists);
           visitCaption(m_t, s);
           m_t << "</div>\n";
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
 
         forceStartParagraph(s);

src/latexdocvisitor.cpp

@@ -473,8 +473,6 @@ void LatexDocVisitor::operator()(const DocVerbatim &s)
           startDotFile(fileName,s.width(),s.height(),s.hasCaption(),s.srcFile(),s.srcLine(),!exists);
           visitChildren(s);
           endDotFile(s.hasCaption());
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
       }
       break;
@@ -488,8 +486,6 @@ void LatexDocVisitor::operator()(const DocVerbatim &s)
         if (!fileName.isEmpty())
         {
           writeMscFile(fileName, s, !exists);
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
       }
       break;

src/rtfdocvisitor.cpp

@@ -368,8 +368,6 @@ void RTFDocVisitor::operator()(const DocVerbatim &s)
           writeDotFile(fileName, s.hasCaption(), s.srcFile(), s.srcLine(), !exists);
           visitChildren(s);
           includePicturePostRTF(true, s.hasCaption());
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
       }
       break;
@@ -385,8 +383,6 @@ void RTFDocVisitor::operator()(const DocVerbatim &s)
           writeMscFile(fileName, s.hasCaption(), s.srcFile(), s.srcLine(), !exists);
           visitChildren(s);
           includePicturePostRTF(true, s.hasCaption());
-
-          if (Config_getBool(DOT_CLEANUP)) Dir().remove(fileName.str());
         }
       }
       break;

src/util.cpp

@@ -6956,3 +6956,13 @@ QCString writeFileContents(const QCString &baseName,const QCString &extension,co
 }
 
 
+void cleanupInlineGraph()
+{
+  if (Config_getBool(DOT_CLEANUP))
+  {
+    for (const auto& fileName: writeFileContents_set)
+    {
+      Dir().remove(qPrint(fileName));
+    }
+  }
+}

src/util.h

@@ -486,5 +486,6 @@ QCString extractBeginRawStringDelimiter(const char *rawStart);
 QCString extractEndRawStringDelimiter(const char *rawEnd);
 
 QCString writeFileContents(const QCString &baseName,const QCString &extension,const QCString &content,bool &exists);
+void cleanupInlineGraph();
 
 #endif