Check cmake <foo>_FOUND variables for existance

Summary:
Otherwise if one passes -DCMAKE_DISABLE_FIND_PACKAGE_foo for all 5
optional dependencies cmake throws an error at you because they all
don't exist: 'if given arguments: "AND" "AND" "AND" "AND" Unknown
arguments specified'

Test Plan:
Error went away, still passes if optional deps aren't
disabled

Differential Revision: https://phabricator.kde.org/D12976