Develop libaec fix

[byrnHDF]

This change allows pre-installed libaec to be used. It does not work for libaec installs that are not in a common root directory. Those will need further work, most likely adding a libaec/SZIP CMake Find module.

Fixes #4614

[lrknox]

Is there is only one *_USE_EXTERNAL:BOOL=OFF option to set? Is it libaec_USE_EXTERNAL:BOOL=OFF, or one or more others?

[lrknox]

Maybe (For libaec use -Dlibaec_* in place of -DSZIP_* for these options) would be clearer, if that is the actual meaning?

[vessokolev]

That is the actual meaning indeed. The original SZIP package (under that name) is not only carrying code with problematic licence, it is very old as a conception. Currently, the only reliable SZIP compressor and encoder is provided by Libaec. Maybe some sites still support SZIP, but I consider that to be driven by purely anthropological interest.

[lrknox]

This command "cmake -C ../<hdf5_src_dir>/config/cmake/cacheinit.cmake -G "Unix Makefiles" -DHDF5_ALLOW_EXTERNAL_SUPPORT:STRING="GIT" -DHDF5_ENABLE_ROS3_VFD=ON -DHDF5_ENABLE_ZLIB_SUPPORT=ON -DHDF5_ENABLE_SZIP_SUPPORT=ON -DHDF5_ENABLE_PLUGIN_SUPPORT=ON -DCMAKE_BUILD_TYPE:STRING=Release ../<hdf5_src_dir>" now downloads and builds szip from https://github.com/MathisRosenhauer/libaec.git using the code from byrnHDF:develop-libaec-fix.

[byrnHDF]

Yes that is true - however as a general rule we try not to force options upon the community.
However I should switch the SZIP to be the "other" choice and make libaec be the example.

[lrknox]

Joe Lee finds it fails on Windows: https://hdfgroup.atlassian.net/wiki/spaces/~55705827c8ac75541347acae79813b105d2b62/blog/2024/12/20/921763895/vcpkg+libaec

[lrknox]

The remaining problem finding system installed libaec seems to occur when 1.) the include directory (containing libaec.h and szlib.h) and the lib directory (with libaec* and libsz*) are not installed in the same location, or when the lib files don't match the pattern beginning with lib (VCPkg on windows). These installs will most likely require a SZip/libaec CMake Find module (issue #5217) as setting libaec_LIBRARY and libaec_INCLUDE_DIR don't currently work when they don't share the same root directory

Otherwise, testing this PR on macOS 10.13 and 15.1, Ubuntu 24.04 and Windows 11:

• building libaec with "sudo make install" per the CMake directions in the INSTALL.md file installs the files in /usr/local on the ubuntu, where it is found when -DHDF5_ENABLE_SZIP_SUPPORT=ON without needing any other CMake options.
• brew install libaec on the older Mac installs it similarly with the same results. I don't have sudo privilege on the newer one.
• For libaec installs in non-system locations, setting libaec_ROOT is sufficient provided the include and lib directories share that common root directory.
• The VCPcg install, where the lib files are named aec.lib and szip.lib includes a libaec-config.cmake file with find_library commands for those filenames, but our current HDF5 CMake code doesn't use it.