Use INSTALL_DIR to customize mruby local installation directory
Use INSTALL_DIR to customize mruby local installation directory

This change effectively makes `make distcheck` succeed.

icing/nghttp2,wzyboy/nghttp2,wzyboy/nghttp2,serioussam/nghttp2,icing/nghttp2,icing/nghttp2,serioussam/nghttp2,thinred/nghttp2,serioussam/nghttp2,icing/nghttp2,icing/nghttp2,wzyboy/nghttp2,icing/nghttp2,thinred/nghttp2,wzyboy/nghttp2,thinred/nghttp2,thinred/nghttp2,serioussam/nghttp2,wzyboy/nghttp2,thinred/nghttp2,thinred/nghttp2,serioussam/nghttp2,serioussam/nghttp2,wzyboy/nghttp2,icing/nghttp2,thinred/nghttp2,serioussam/nghttp2,wzyboy/nghttp2