Report details about fork() failure in logs.

When parsing a large codebase dot failed to start. Adding this line helped narrow down the issue. Increasing my VirtualBox VM RAM size worked around the issue. A proper fix would be to spawn separate processes without using fork() so they don't start with the giant address space used by the current process.

Bug 715172 - Consistently document Fortran's equivalent function bodies

Bug 719639 - @xrefitem with empty heading string

Some texts can be independent of \htmlonly / \latexonly

Bug 668424 - HTML links in RTF output are broken

Bug 719489 - Label "abstract" instead of "pure virtual" for Java language

Proposed patch so the word abstract is recorded correctly in case of Java

There are different types of hyperlinks: external and internal. The internal hyperlinks should have \\l behind the word HYPERLINK, the external shouldnt.

Replaced:
  if (!(x->title().length())) return;
by:
  if (x->title().isEmpty()) return;

Adjusted documentation and in case the title is the empty string in a DocXRefItem the visitPre and visitPost do nothing.

Fortran added keyword IMPURE

Added keyword IMPURE analogous to keyword PURE

Although controversioal but for consistency converted type for fortran functions to lowercase

Bug 719525 - Can't build because the definitions of ***YY_flex_debug are...

Update the documentation in respect to the version of flex and bison that can be used.

Conflicts:
	src/memberdef.cpp
	src/memberdef.h

Debug output for lexical analyzer