This could remove recurring of the same statement for include directories.
Instead of this removals, apps/* and tests should have include path that
had been provided by INTERFACE_INCLUDE_DIRECTORIES of target libs and
internal static libs.
Internally LIBRESSL_SKIP_INSTALL, if not set becomes ENABLE_LIBRESSL_INSTALL so this by default is enabled. defining LIBRESSL_SKIP_INSTALL before hand will disable all install() rules.
This is useful if another project includes and links to this statically.
I chose to add a prefix to avoid potential name collision because the options are cached globally.
If the installation is skipped, maybe it should also disable building apps? I didn't do that.
