Fix building with cmake on windows

[oyvindln]

Neither qwt nor pkg-config seems to include a .pc file on windows, so this borrows a findQwt script from QGis to make handling it easier.

The other addition needed was to define _USE_MATH_DEFINES as M_PI is a non-standard extension and as such not available from <cmath> when compiling with MSVC otherwise.

Now builds with cmake+msbuild on windows if the needed packages are installed via vcpkg and using the ms dev environment command line and vcpkg cmake toolchain file.

ld-ldf-reader is still disabled for windows as it uses some linux/unix-specific functions, also disable it for macos as @kaliohelix on discord reported it didn't build properly there either.

[atsampson]

Please only fall back to FindQwt if pkg-config doesn't work (and to be honest, I think you'd do better to report a bug in the Windows packaging of Qwt you're using if they aren't providing the .pc file). As we found when testing this across different Linux distributions, FindQwt's approach of guessing wildly at installation directories does not work reliably.

[oyvindln]

Ok, it should now try with pkg-config first.

Which distros does it fail on? It's a different find script than what was used last previously.

It looks like qwt explicitly disables pkg-config generation on non-UNIX systems:
https://sourceforge.net/p/qwt/git/ci/develop/tree/qwtconfig.pri
Also Qt 6.0 and 6.1 lacked some pkg-config stuff. It's possible the config to enable it for qwt works on win but I don't know.

[oyvindln]

Also changed the conditional for ld-ldf-reader, seems the LINUX variable was added only in the last version.

[oyvindln]

bump @atsampson