#79 CMake support

[Michel Zou]

hello,

here is an attempt at providing cmake build support for qwt (against the develop branch)
to facilitate windows and cross-compile builds

it builds on win/linux and qt5/qt6
note that it is not feature-complete (no tests/doc/playground/designer plugin)

it can also be fetched here:
https://github.com/xantares/qwt-cmake