Description of problem:

It is not obvious how to adapt the luminance-hdr package to build with the new OpenEXR 3.x packages coming soon to Fedora Rawhide (35).

How reproducible:


Steps to Reproduce:
1. 

mkdir -p ~/.config/mock
cat > ~/.config/mock/fedora-rawhide-openexr3.cfg <<'EOF'
include('/etc/mock/fedora-rawhide-x86_64.cfg')

config_opts['root'] = 'fedora-rawhide-openexr3'

config_opts[f'{config_opts.package_manager}.conf'] += """

[openexr3]
name=Copr repo for openexr owned by hobbes1069
baseurl=https://download.copr.fedorainfracloud.org/results/hobbes1069/openexr/fedora-rawhide-$basearch/
type=rpm-md
skip_if_unavailable=True
gpgcheck=1
gpgkey=https://download.copr.fedorainfracloud.org/results/hobbes1069/openexr/pubkey.gpg
repo_gpgcheck=0
enabled=1
enabled_metadata=1
EOF

2. fedpkg co luminance-hdr
3. cd luminance-hdr
4. fedpkg srpm
5. mock -r fedora-rawhide-openexr3 --rebuild luminance-hdr-2.6.1.1-3.fc35.src.rpm

Actual results:

Build fails

Expected results:

Build succeeds

Additional info:

I tried removing the lines

```cmake
FIND_PACKAGE(OpenEXR REQUIRED)
INCLUDE_DIRECTORIES(${OPENEXR_INCLUDE_DIR} "${OPENEXR_INCLUDE_DIR}/OpenEXR")
```

from `CMakeLists.txt` to patch out the OpenEXR 2.x specific configuration from upstream. Instead, I added the output of `pkgconf --cflags OpenEXR` to the global `CFLAGS` and `CXXFLAGS`, and the output of `pkgconf --libs OpenEXR` to `LDFLAGS`.

The result was a panoply of linker errors.

I inferred that the problem might be that openexr was built using the compiler’s default C++ standard, which is gnu++17, while LuminanceHDR forces C++11. There is no (reliable) ABI compatibility across C++ versions, so I patched `CMAKE_CXX_STANDARD 17` (instead of 11) into `build_files/Modules/CompilerSettings.cmake`; just setting `-DCMAKE_CXX_STANDARD=17` did not override the contents of this file.

Switching to C++17 meant `std::clamp` appeared in `<algorithm>`, which was ambiguous with the `clamp` in an unnamed namespace in `src/Libpfs/manip/gamma_levels.cpp`. To resolve this, I could either remove the definition of `clamp` and add `using std::clamp;` (or call `std::clamp(` every time), or rename `clamp`, e.g. to `xclamp`, and call that instead. I chose the former.

After all that, I was back to staring at a bunch of linker errors of the form:

> undefined reference to `Imf_3_0::…`

even though

> -lOpenEXR-3_0 -lOpenEXRUtil-3_0 -lIex-3_0 -lIlmThread-3_0 -lImath-3_0

are in the linker invocation.

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I am not sure I will be able to fix this with a reasonable amount of effort. Help is more than welcome.