libressl is NOT portable. Supporting BSD and Linux is not the definition of "portable" (see also: "We play both types of music: Country and Western"). The libressl code depends on the non-standard #include_next preprocessor directive, so it can only build with GCC (and probably clang, which emulates many GCC-isms). Forget about building on Windows using Microsoft's C compiler.
OpenSSL remains the only portable SSL library that can be used by both open source and commercial developers alike. Which is really a shame, because OpenSSL sucks. All the bad things the libressl people have said about OpenSSL are absolutely true.