mod_ssl: build with LibreSSL.

LibreSSL seems to be openssl-1.1 API compatible only in version 2.8 (master).
So use that for MODSSL_USE_OPENSSL_PRE_1_1_API instead of 2.7, the two 2.7
compatibility-exceptions are handled explicitely but overall it's simpler.

Regarding CRYPTO_malloc_init vs OPENSSL_malloc_init, libreSSL uses none, the
former used to be a no-op but depends is LIBRESSL_INTERNAL in latest versions,
while the latter has never been (and will never be) defined. So don't call any
with LibreSSL.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1833598 13f79535-47bb-0310-9956-ffa450edef68