Merge branch 'set_nghttp2_debug_callback' of https://github.com/Andersbakken/nghttp2 into Andersbakken-set_nghttp2_debug_callback

when building DEBUGBUILD.

Make it possible to include nghttp2/CMakeLists.txt in another project

using add_subdirectory.

CMAKE_SOURCE_DIR/CMAKE_BINARY_DIR points to the top level
CMakeLists.txt. This isn't necessarily nghttp2/CMakeLists.txt.

We have now generic read-only command execution in shrpx_exec.{h,cc}.

This option requires OpenSSL >= 1.0.2.  With OpenSSL 1.0.2, the
default value is "P-256".  With OpenSSL 1.1.0 or later, the default
value is "X25519:P-256".