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.
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