Please help with qelectrotech 0.8.0 cmake for Debian

[Martin]

Dear Laurant, dear Denis, dear science team,

I took a look into QElectroTech 0.8.0, released 2021-02-21, to
update the package in Debian (not targeted for Debian 11
bullseye, of course). See (1) and (2)

Version 0.8.0 uses a cmake feature, that has not been used in
0.7.0, FetchContent_Declare, which seems to try downloads of
library code from various git repositories during build.

Of course, this is a strict no-go in Debian and most other
distributions. Also, it is not necessary, because all the needed
packages are present in Debian, as far as I can tell. (3)

I have to confess, that I don't know anything about cmake, and
my naive attempts of just using "DOWNLOAD_COMMAND true" and
removing the GIT_ directives failed.

Therefore I ask for your help in correctly building the package
for Debian (and other distributions) by using the existing
Debian packages instead of using FetchContent_Declare.

I'm willing to apply any patches, to test results, and to
upload, but I don't have the capacity to learn cmake and the
subtleties of the Qt ecosystem. Therefore I need some help!

Many thanks in advance for any support in that matter!

Cheers

(1) https://tracker.debian.org/pkg/qelectrotech
(2) https://salsa.debian.org/science-team/qelectrotech
(3) catch2, extra-cmake-modules, googletest, libpugixml-dev,
    libsingleapplication-dev, and ls5-dev