1.15.0 CMake build failure with -DENABLE_PDF:BOOL=FALSE

**Describe the bug**
Trying to build poco 1.15.0 with `-DENABLE_PDF:BOOL=FALSE` and not having libpng available fails with:

```
[...]
CMake Error at dependencies/hpdf/CMakeLists.txt:21 (target_link_libraries):
  Target "_BUNDLED_HPDF" links to:
    PNG::PNG
  but the target was not found.  Possible reasons include:
    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.
CMake Generate step failed.  Build files cannot be regenerated correctly.
```

**To Reproduce**
Pass `-DENABLE_PDF:BOOL=FALSE` and do not have libpng installed.

**Expected behavior**
Build without failing like the previous version 1.14.2 does.

**Logs**
[poco-poco-1.15.0-release_build_job.log](https://github.com/user-attachments/files/25221959/poco-poco-1.15.0-release_build_job.log)

**Please add relevant environment information:**
 - OS Type and Version: Exherbo Linux
 - POCO Version: 1.15.0
 - Third-party product: -

**Additional context**
* should probably also not check for sqlite when building with `-DENABLE_DATA_SQLITE:BOOL=FALSE` which can also be seen in the attached log.
* https://github.com/pocoproject/poco/blob/main/CMakeLists.txt#L279-L288 is quite misleading as well when not using the bundled deps at all as it reads using `Using internal zlib, pcre2, expat, libpng, ...`  but still (correctly) checks for the external/unbundled ones:

```
-- Using external sqlite
-- Using internal zlib, pcre2, expat, libpng, ...
-- Found OpenSSL: /usr/x86_64-pc-linux-gnu/lib/openssl-3/lib/libcrypto.so (found suitable version "3.5.5", minimum required is "1.1.1")
-- Found ZLIB: /usr/x86_64-pc-linux-gnu/lib/libz.so (found version "1.3.1")
-- Found PCRE2: /usr/x86_64-pc-linux-gnu/lib/libpcre2-8.so
-- Found Utf8Proc: /usr/x86_64-pc-linux-gnu/lib/libutf8proc.so
-- Found SQLite3: /usr/x86_64-pc-linux-gnu/include (found version "3.51.2")
CMake Warning at dependencies/sqlite3/CMakeLists.txt:11 (message):
  Unbundled SQLite3: Assuming that it was compiled with SQLITE_THREADSAFE=1
-- Found EXPAT: /usr/x86_64-pc-linux-gnu/lib/libexpat.so (found version "2.7.4")
-- Could NOT find PNG (missing: PNG_LIBRARY PNG_PNG_INCLUDE_DIR) 
```

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