Log Message:
nghttp2: Add GCC_REQ for c++14

While the package without options does not actually use C++14,
configure looks for it, and fails to find the C++ version of the C
preprocessor, at least on betbsd-7 with gcc 4.8.5.

(Perhaps the cpp searching could be fixed, and the USE_LANGUAGES pushed
into options that need it, but that seems more complicated than is
useful, given that one more or less needs to have a C++14 cpmpiler
anyway.)