Broken path resolution for internal CMake macros

[adambarta]

Specifically, the build system relies on a variable called H5PL_RESOURCES_DIR to find its core logic files (like
H5PLMacros.cmake). In the version checked out (git clone ; git checkout tags/hdf5-1.14.6), this variable was failing to auto-detect correctly, resulting in an empty string. This caused two critical failures:

1. The "Leading Slash" Error: include(/H5PLMacros.cmake) failed because it was looking at the root of your hard
   drive instead of the config/cmake folder.
2. Undefined Macros: Because the include failed, the BASIC_SETTINGS and FILTER_OPTION commands were never defined,
   leading to the "Unknown CMake command" errors.

The fix was manually "hardcoding" that path on the command line:
-DH5PL_RESOURCES_DIR=/build/hdf5/hdf5_plugins/config/cmake

This bypassed the broken auto-detection and allowed CMake to finally see the instructions it needed to configure
the respective plugins.