build: support LibreSSL native crypto lib with ngtcp2 1.15.0+

In ngtcp2 1.15.0 the LibreSSL crypto interface library got its own name:
`libngtcp2_crypto_libressl`. In previous versions it used
`libngtcp2_crypto_quictls`, shared with quictls itself (but not
compatible with).

Adapt autotools and cmake scripts to look for the new name first, and
fall back to the old one if not found.

Fallback to quictls tested OK in CI with both autotools and cmake:
https://github.com/curl/curl/actions/runs/17174994908?pr=18377

Ref: https://github.com/ngtcp2/ngtcp2/releases/tag/v1.15.0
Ref: https://github.com/ngtcp2/ngtcp2/pull/1716

Closes #18377

m4/curl-openssl.m4

@@ -292,6 +292,7 @@ if test "x$OPT_OPENSSL" != xno; then
     ],[
       AC_MSG_RESULT([yes])
       ssl_msg="LibreSSL"
+      HAVE_LIBRESSL=1
     ],[
       AC_MSG_RESULT([no])
     ])