generalize make_windows_release for MinGW or MSVC. (#151)

generalize make_windows_release for MinGW or MSVC. (#151)

make_windows_release.ps1 now can be used with MinGW or MSVC.  It is likely we will have to switch compilers with Qt  >= 5.6 because MinGW is not supported with webengine, and Qt >= 5.6 release packages from www.qt.io distributes webengine and not webkit (although some other parties build webkit from source with these versions of Qt).
appveyor.yml is modified to run a release on one MinGW case (Qt 5.5, webkit) and one MSVC case (Qt 5.6, webengine, 32 bit).  In the case of MSVC the VC redistributible installer is run by setup.
Note that inno setup compiler is always run in 32 bit mode, so if you use this with 64 bit executables they will install in the wrong place, i.e. "Program Files (x86)"