Log Message:
nghttp2: Rototill USE_LANGUAGES and make granular per option

Back out the previous build fix.

The base package does not even need C++, so drop from USE_LANGUAGES,
and remove GCC_REQD.  Builds on netbsd-7/amd64 with gcc 4.8.

Both options build C++ code.  Add c++14 to USE_LANGUAGES and 6 to
GCC_REQD, following upstream documentation.