CMake: Fix QDoc configure warnings not shown

Since 33d1e3727, _qdoc_add_configure_warning() received the condition
as one quoted string. The configure condition evaluator expects one
token per list element, so it looked up a variable named like
"NOT QT_FEATURE_clang", which is never defined. None of the QDoc
warnings were shown, neither in per-repo nor in top-level builds.
Pass the condition as separate arguments instead.

The libclang version warning referred to QDOC_MINIMUM_CLANG_VERSION,
a non-cache variable of qttools' directory scope. A top-level build
evaluates report conditions at the very end in the top-level scope,
where the variable is undefined, so that warning was missing there.
Expand the value when registering the warning. Also only show that
warning if libclang was found at all, because otherwise the "LLVM/Clang
libraries could not be located" warning already applies.

Fixes: QTBUG-123981
Pick-to: 6.12
Change-Id: I566b6c6658c4e4b9a03c85f723709576fb132011
Reviewed-by: Paul Wicking <paul.wicking@qt.io>