"LibreSSL 2.4.x support has also ended."

[concatime]

Since November 6, the old 2.4.x is deprecated. We need to update to 2.6.3, or actually consider unbundling it, reference to #1461.
I would love to provide a commit, but It seems that you rely on a proper .tar.gz file instead of downloading it.
Source.

[kazuho]

Thank you for opening the issue.

We will unbundle libressl in upcoming 2.3 series. We will continue to use libressl-2.4.6 for 2.2 series to preserve NPN support; it has been dropped in 2.6.3.

[concatime]

Concerning NPN, can I currently use H2O 2.2.3 with LibreSSL 2.6.3?

[kazuho]

I think you can. You might need to set PKG_CONFIG_PATH to the <PREFIX>/lib/pkgconfig where PREFIX is your OpenSSL / liberssl installation directory, or use a more specific way (i.e. those descirbed in https://cmake.org/cmake/help/latest/module/FindOpenSSL.html#hints).