Provide support for Cygwin 64-bit

Recent versions of Cygwin include a 64-bit version, installed under c:\cygwin64 by default.

We should support this configuration, at least in two places:

• when compiling MEX files with use_dll; see http://www.dynare.org/DynareWiki/ConfigureMatlabWindowsForMexCompilation
• in README.md, for people compiling from source