nghttp2: Requires C++14

[ryandesign (Ryan Carsten Schmidt)]

​nghttp2 fails to build on 10.9:

checking whether /usr/bin/clang++ supports C++14 features with -std=c++14... no
checking whether /usr/bin/clang++ supports C++14 features with +std=c++14... no
checking whether /usr/bin/clang++ supports C++14 features with -h std=c++14... no
checking whether /usr/bin/clang++ supports C++14 features with -std=c++1y... no
checking whether /usr/bin/clang++ supports C++14 features with +std=c++1y... no
checking whether /usr/bin/clang++ supports C++14 features with -h std=c++1y... no
configure: No compiler with C++14 support was found

./template.h:45:16: warning: variable templates are a C++1y extension [-Wc++1y-extensions]

nghttp2 already includes the cxx11 1.1 portgroup, but since it requires C++14, it should additionally use:

PortGroup compiler_blacklist_versions 1.0

# Requires C++14
compiler.blacklist-append {clang < 602}

[Schamschula (Marius Schamschula)]

In 4fc3e723bfe0613d25d94480aef607a102df5b36/macports-ports (master):

nghttp2: require C++14

Closes: #57682