\usepackage{fixltx2e} must come before use \usepackage{float}

The \usepackage{doxygen} statement adds float as a depenency.
If the order is wrong the latex generation fails with:
LaTeX Error: Unknown float option `H'
Bug 727732 - Nested C structures/unions does not work with groups
Bug 731238 - Dead links in html documentation when using tabs for indentation in c++
Bug 730887 - [PATCH] Fix potential copy and paste error in inputstring.cpp
Merge pull request #178 from ellert/master

\usepackage{fixltx2e} must come before use \usepackage{float}
Merge branch 'master' of github.com:doxygen/doxygen