Summary

Fixes #5196: CMake build failure when using POCO_UNBUNDLED=ON with features like PDF disabled (-DENABLE_PDF=OFF) and their corresponding system libraries not installed.

• dependencies/png, expat, sqlite3: When unbundled, only find_package() for libraries that are actually needed by enabled features. Previously, disabled features still triggered unnecessary (and sometimes failing) find_package() calls.
• dependencies/hpdf: Early return() with warning when POCO_UNBUNDLED is set and PNG::PNG target doesn't exist, preventing the target_link_libraries error on a missing target.
• CMakeLists.txt: Fix status message logic — POCO_UNBUNDLED forces POCO_SQLITE_UNBUNDLED=ON, so checking POCO_SQLITE_UNBUNDLED inside the POCO_UNBUNDLED branch was always true. Restored the correct 3-branch structure.
• CMakeLists.txt: Force ENABLE_FOUNDATION=ON since all components depend on it.

Test plan

• cmake -DENABLE_PDF=OFF -DPOCO_UNBUNDLED=ON succeeds without libpng installed
• cmake -DENABLE_DATA_SQLITE=OFF -DPOCO_SQLITE_UNBUNDLED=ON succeeds without sqlite warning
• cmake -DENABLE_XML=OFF -DPOCO_UNBUNDLED=ON succeeds without libexpat installed
• cmake -DENABLE_PDF=ON -DPOCO_UNBUNDLED=ON correctly requires system libpng
• cmake -DENABLE_FOUNDATION=OFF builds Foundation anyway
• Default bundled build unaffected
• Tested 111 option combinations across bundled, sqlite-unbundled, and fully unbundled modes with 0 failures