Conan package does not support doctest integration

[WBurzynski]

options.integration does not accept doctest value.

Steps to reproduce:

1. create conanfile.txt with following option:

[options]
fakeit/2.4.1:integration=doctest

2. Run conan install

expected outcome:
conan install will succedd with doctest integration configured.

actual outcome:
conan install fails with following error:

ERROR: 'doctest' is not a valid 'options.integration' value.
Possible values are ['boost', 'catch', 'cute', 'gtest', 'mettle', 'nunit', 'mstest', 'qtest', 'standalone', 'tpunit']

I patched the package locally and it worked, but I had to change include from doctest.h to doctest/doctest.h. I'm willing to open a PR, but I don't know how breaking that change would be.

[FranckRJ]

It's because doctest isn't available for all fakeit versions, so it's just about adding the option in the conan package but also making that option only available on newer versions of fakeit. I don't know if it's something you can do with a simple if in the recipe or if you have to do another version of the recipe (which is annoying), so I did nothing about it yet.