Satisfy the OpenSSF Best Practices Badge requirement for an
insta/uninstall process:
https://www.bestpractices.dev/en/criteria/1#1.installation_common

CMake does not support a standard "uninstall" target, but the
community recommends implementing an "uninstal" target that remove files named in the
`install_manifest.txt`:
https://gitlab.kitware.com/cmake/community/-/wikis/FAQ#can-i-do-make-uninstall-with-cmake

However, our existing process of installing the symlink to the "bare"
library, i.e. the symlink from libImath-3_2.so to libImath.so, fails
to add the symlink to the manifest, so "make uninstall" misses the
symlink. The existing mechanism use "install(CODE execute_process(cmake -E create_symlink))".

This changes that to use a simpler "file(CREATE_LINK)" and
"install(FILES)" to accomplish the same thing while also registering
the symlink the the manifest.

Also, this fixes an issue where `OpenEXRConfig.h` was passed to
`install()` twice, producing two entries in `install_manifest.txt`.

Signed-off-by: Cary Phillips <cary@ilm.com>

Signed-off-by: Cary Phillips <cary@ilm.com>

Signed-off-by: Cary Phillips <cary@ilm.com>

Signed-off-by: Cary Phillips <cary@ilm.com>

Signed-off-by: Cary Phillips <cary@ilm.com>