Table of contents breaks when documentation spans multiple comment bl…

…ocks with same @page

Based on the question in the doxygen users forum:
http://doxygen.10944.n7.nabble.com/Table-of-contents-breaks-when-documentation-spans-multiple-comment-blocks-with-same-page-td7571.html

Fixed by means of that when toc is set once for a page it remains set for that page, otherwise the last page has to have the @TableofContents command (or all @page commands)

src/pagedef.cpp

@@ -308,6 +308,6 @@ void PageDef::setNestingLevel(int l)
 
 void PageDef::setShowToc(bool b)
 {
-  m_showToc = b;
+  m_showToc |= b;
 }