Solve use_dll-incompatibility problem on Windows in master

After merging the temporary_terms-branch (https://github.com/DynareTeam/dynare/commit/127637ffd672fdd8143ee13f2a38234e597d94d3) the c-files created are not C89-compatible, but require C99. This is a problem for Windows machines as Visual Studio before version 2015 only supported C89, resulting in errors of the form error C2143: syntax error : missing ';' before 'type' Before Matlab version R2014b users could use the cygwin-option by using the provided mexopts-win64.bat from http://www.dynare.org/DynareWiki/ConfigureMatlabWindowsForMexCompilation, which should not have this restriction. Since R2015b, mingw is available, which also supports C99 (#1226 (closed)). Moreover, Visual Studio 2015 finally supports C99 and has a free available version that is supported since R2015b. That leaves 64bit Matlab-versions between R2014a and R2015a unable to deal with the problem unless someone figures out how to solve #641 (closed) for cygwin (potentially following the advice on how to set up the xml-file at http://stackoverflow.com/questions/8552580/using-gcc-mingw-as-matlabs-mex-compiler). Given that there seems to be not that much demand for use_dll on Windows, I would propose for 4.5 to focus on the mingw-support and state the above restrictions clearly in the release notes and the manual