cmake: Conditionally add log and log_setup to exported configuration

[rleigh-codelibre]

These were missing from the exported configuration. Partly an oversight, and partly because they were conditionally used. This adds them conditionally so should work in all cases.

Testing: Check all builds remain green. Check OMECommonConfig.cmake and ensure that it contains:

find_dependency(Boost 1.46 REQUIRED
                COMPONENTS date_time filesystem system iostreams
                program_options regex log_setup log)

(in any build; CentOS7 platform builds will have it missing because Boost.Log is not available, but we don't have this in the CI builds yet.)

[sbesson]

@rgozim did you have a chance to review this PR and its companion ones? Do these need to be relisted for review?

[rgozim]

Everything looks okay. CI box build looks to working too, except for a small error due to a missing header file in OME-FILES-CPP-DEV-merge-sourcebuild.

[sbesson]

Is the drop of the REQUIRED qualifier required for the conditional import to work or is this related to the find_dependency limitations discussed in ome/ome-files-cpp#85 (comment)?

[rleigh-codelibre]

It's unrelated to both. It's always been incorrect to use REQUIRED with find_dependency, but I didn't understand this at the time these files were written. find_dependency adds REQUIRED for you if the top-level find_package call used REQUIRED otherwise it's not added and left optional. It will work without the change, but the behaviour will be wrong.