[eve7] Include json_fwd.hpp if available

[hahnjo]

Version 3.11 of nlohmann/json introduced "versioned, ABI-tagged inline namespace"s, which breaks our forward declaration. Fortunately, we can assume the json_fwd.hpp header to be present starting from that same version because the JSON_MultipleHeaders option now defaults to ON and even if not, json_fwd.hpp is installed since patch version 3.11.2. For
earlier versions, both methods work but json_fwd.hpp isn't guaranteed to be installed. Still use it if available.

Fixes #11130

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14
How to customize builds

[linev]

json_fwd.hpp exists since a while and included in many early distributions.
My OpenSUSE has nlohmann/json.hpp version 3.10.5 with such forward declaration.

Problem is ROOT builds where nlohmann/json.hpp can be found only with extra settings for shell variables.
In such cases none of its includes can be used in public ROOT interfaces.

[falbrechtskirchinger]

Some notes:

• Avoid v3.11.0 and v3.11.1 if possible, i.e., advise users to upgrade.
• We did not split the inline namespace in v3.11.2 (it's still just one).
• json_fwd.hpp is only available from v3.11.0 if built with -DJSON_MultipleHeaders=ON.
• If not for the inline namespace, this would have broken once nlohmann/json#3110 is merged.

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14
How to customize builds

[hahnjo]

json_fwd.hpp exists since a while and included in many early distributions. My OpenSUSE has nlohmann/json.hpp version 3.10.5 with such forward declaration.

Problem is ROOT builds where nlohmann/json.hpp can be found only with extra settings for shell variables. In such cases none of its includes can be used in public ROOT interfaces.

I know, but we have to do something, and as agreed on Wednesday, we actually want to ensure that users have the same version of nlohmann/json.

[linev]

eve7 examples working - tested with opensuse and system-wide install json.hpp and json_fwd.hpp
One can merge it!

[wdconinc]

Does this merit a backport into 6.26 branch since it prevents compilation? Just wondering from a packager's point of view keeping an eye on build issues.

[hahnjo]

@wdconinc yes, I already posted #11225 (currently waiting for Jenkins to finish before merging). Will be released as 6.26/08 (no date planned yet AFAIK), or you can pick the middle commit yourself.