I can get the package to build using the following steps:

git clone https://github.com/nomacs/nomacs.git
cd nomacs
mkdir build
cd build
guix environment --ad-hoc cmake make gcc libraw exiv2 libtiff opencv python
quazip qtbase qtsvg qttools pkg-config git
cmake "../ImageLounge" "-DCMAKE_BUILD_TYPE=RelWithDebInfo"
"-DCMAKE_INSTALL_PREFIX=/gnu/store/fiwxb6mlwhvnr49f9ydbhbam\
wp4bzb8b-nomacs-3.16.224" "-DCMAKE_INSTALL_LIBDIR=lib"
"-DCMAKE_INSTALL_RPATH_USE_LINK_PATH=TRUE" "-DCMAKE_INSTALL_RPA\
TH=/gnu/store/fiwxb6mlwhvnr49f9ydbhbamwp4bzb8b-nomacs-3.16.224/lib"
"-DCMAKE_VERBOSE_MAKEFILE=ON" "-DENABLE_TRANSLATIO\
NS=true" "-DUSE_SYSTEM_QUAZIP=true" "-DENABLE_OPENCV=true"
make -j 8
./nomacs

For some reason that is different than when `guix package --from-file` runs.

Den fre 1 jan. 2021 kl 23:02 skrev Rovanion Luckey <
rovanion.luckey@gmail.com>:

> Hi Leo and thank you for explaining. And hi again guix-devel.
>
> I've now rewritten the file according to your comments except that its
> still its own file/module, something I'll fix later.
>
> Now I'm stuck trying to figure out a build error after going from the
> tarball release through url-fetch to the non-recursive git-clone download.
> CMake finds the package OpenCV when it runs, removing OpenCV from the
> package inputs results in an error from CMake (as expected). But the build
> fails during the build phase with the following error:
>
> > make[2]: *** No rule to make target 'opencv_core-NOTFOUND', needed by
> 'libnomacsCore.so.3.16.0'.  Stop.
>
> I've not found any relevant mentions of opencv_core in the source code. I
> tried adding monkeying about with the CMakeLists a little to no avail. Also
> looked at what Debian does to their package but found nothing that I
> thought to be relevant:
> https://sources.debian.org/src/nomacs/3.12.0+dfsg-3/debian/
>
> I've attached the new package source.
>