Fix INSTALL_CMAKE_DIR for GNU_DIRS and enable by default on Linux.

[chuckatkins]

Describe your changes

The previous value of HDF5_INSTALL_CMAKE_DIR of lib/cmake when HDF5_USE_GNU_DIRS=ON isn't in CMake's find_package search path since CMake expects that to be a common path with each package having it's own subdirectory:

Entry	Convention
<prefix>/<name>/cps/	W
<prefix>/<name>/*/cps/	W
<prefix>/cps/<name>/	W
<prefix>/cps/<name>/*/	W
<prefix>/cps/	W
<prefix>/	W
<prefix>/(cmake|CMake)/	W
<prefix>/<name>*/	W
<prefix>/<name>*/(cmake|CMake)/	W
<prefix>/<name>*/(cmake|CMake)/<name>*/	W
<prefix>/(lib/<arch>|lib*|share)/cps/<name>/	U
<prefix>/(lib/<arch>|lib*|share)/cps/<name>/*/	U
<prefix>/(lib/<arch>|lib*|share)/cps/	U
<prefix>/(lib/<arch>|lib*|share)/cmake/<name>*/	U
<prefix>/(lib/<arch>|lib*|share)/<name>*/	U
<prefix>/(lib/<arch>|lib*|share)/<name>*/(cmake|CMake)/	U
<prefix>/<name>*/(lib/<arch>|lib*|share)/cmake/<name>*/	W/U
<prefix>/<name>*/(lib/<arch>|lib*|share)/<name>*/	W/U
<prefix>/<name>*/(lib/<arch>|lib*|share)/<name>*/(cmake|CMake)/	W/U

This changes HDF5_INSTALL_CMAKE_DIR to lib/cmake/hdf5 when HDF5_USE_GNU_DIRS=ON so the CMake package config is now found when the install prefix is in CMAKE_PREFIX_PATH.
This also sets HDF5_USE_GNU_DIRS=ON by default for non-Windows builds.

Issue ticket number (GitHub or JIRA)

Fixes #6137

[chuckatkins]

It seems the Windows / not windows default isn't working as expected. Debugging it now, will push a fix shortly

[jhendersonHDF]

Thanks for these changes @chuckatkins. We probably need to do some more testing but this generally looks fine; I think my only concern is enabling HDF5_USE_GNU_DIRS for all non-Windows. I'm aware of CMake's table for where find_package looks, though just having W/U/A doesn't clarify much about where CMake will search on non-Windows, non-Unix, non-Apple platforms.