Add an APIARY_BUILD_TOOL option (default ON) that gates the libtooling
executable separately from the header-only annotation contract.

Previously, descending into this CMakeLists always ran
find_package(Clang/LLVM) and built the apiary executable, and the
apiary_annotations target + its header install only happened as a
side effect of that descent. A consumer that wanted just the APIARY_*
macros (every annotated C++ TU) was forced to pull in the full
Clang/LLVM dev stack, and gating the descent to avoid that silently
dropped the annotation-header install too — leaving find_package(Apiary)
and install trees without apiary/Annotations.hpp.

Now:
  * apiary::annotations (INTERFACE) and the include/apiary header install
    are unconditional — no Clang/LLVM required.
  * find_package(Clang/LLVM), the apiary executable, its links, alias,
    and the tool-driven smoke/golden/pyi tests are guarded by
    APIARY_BUILD_TOOL. The pure-Python doc_lint test still runs either way.
  * The executable joins the ApiaryTargets export only when built;
    ApiaryConfig sets APIARY_TOOL_FOUND and documents that apiary::apiary
    may be absent in a header-only install.

This lets a consumer (Einsums) vendor the annotation macros in a pure-C++
build while enabling the tool only for bindings generation or docs.

Verified: APIARY_BUILD_TOOL=OFF configures and installs with no
Clang/LLVM and ships include/apiary/Annotations.hpp; =ON finds LLVM and
builds the executable as before.

Signed-off-by: Justin Turney <justin.turney@gmail.com>