Add CMake build mode flags to hdf5lib.settings

[derobins]

Flags from the CMake build mode (e.g., optimization) are not a part of CMAKE__FLAGS and were not exported to the libhdf5.settings file. This has been fixed and the C, Fortran, and C++ build mode flags are now exported to the file.

This also affects the text output of H5check_version() and the libhdf5.settings string stored in the library (for those who use strings(1), etc. to get build info from the binary).