conditional find_package()

[tclune]

We now have a few instances where a package is only "mostly" required. E.g., pflogger now has a stub option.

A cmake macro tha wraps find_package() could perhaps make the logic more clear. It would take the same options as find_package() but with an additional argument that controls whether to invoke find_package()

esma_find_package(PFLOGGER REQUIRED IF ${BUILD_WITH_PFLOGGER})

Not 100% certain that this is worthwhile though. Comments appreciated.
@weiyuan-jiang @rmontuoro @mathomp4