problems building ruby-capybara-webkit

I've been hitting this recently. (Standard bulk build with gcc on i386,
also same if I test on a normal amd64 workstation).

rm -f *~ core *.core
current directory: /usr/obj/ports/capybara-webkit-1.11.1-ruby24/gem-tmp/.gem/ruby/2.4/gems/capybara-webkit-1.11.1
make V=1 "DESTDIR="
cd src/ && ( test -e Makefile.webkit_server || /usr/local/bin/qmake-qt5 /usr/obj/ports/capybara-webkit-1.11.1-ruby24/gem-tmp/.gem/ruby/2.4/gems/capybara-webkit-1.11.1/src/webkit_server.pro -o Makefile.webkit_server ) && make -f Makefile.webkit_server
c++ -pipe -O2 -std=gnu++0x -Wall -W -pthread -fPIC -DQT_NO_DEBUG -DQT_WEBKITWIDGETS_LIB -DQT_WIDGETS_LIB -DQT_WEBKIT_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -isystem /usr/local/lib/gcc/i386-unknown-openbsd6.1/4.9.4/include -I. -isystem /usr/local/include/X11/qt5 -isystem /usr/local/include/X11/qt5/QtWebKitWidgets -isystem /usr/local/include/X11/qt5/QtWidgets -isystem /usr/local/include/X11/qt5/QtWebKit -isystem /usr/local/include/X11/qt5/QtGui -isystem /usr/local/include/X11/qt5/QtNetwork -isystem /usr/local/include/X11/qt5/QtCore -Ibuild -I/usr/X11R6/include -isystem /usr/local/include -I/usr/local/lib/qt5/mkspecs/openbsd-g++ -x c++-header -c stable.h -o build/webkit_server.gch/c++
cc1plus: error: unrecognized command line option "-std=gnu++0x"
*** Error 1 in src (Makefile.webkit_server:980 'build/webkit_server.gch/c++')
*** Error 1 in /usr/obj/ports/capybara-webkit-1.11.1-ruby24/gem-tmp/.gem/ruby/2.4/gems/capybara-webkit-1.11.1 (Makefile:46 'sub-src-webkit_server-pro-make_first-ordered')
ERROR: Error installing /usr/obj/ports/capybara-webkit-1.11.1-ruby24/capybara-webkit-1.11.1.gem:
ERROR: Failed to build gem native extension.

Building has failed. See above output for more information on the failure.
make failed, exit code 1

...

If I change it to use ports gcc, I get a different failure:

c++ -c -include build/webkit_server -pipe -O2 -std=gnu++0x -Wall -W -pthread -fPIC -DQT_NO_DEBUG -DQT_WEBKITWIDGETS_LIB -DQT_WIDGETS_LIB -DQT_WEBKIT_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -isystem /usr/local/lib/gcc/x86_64-unknown-openbsd6.1/4.9.4/include -I. -isystem /usr/local/include/X11/qt5 -isystem /usr/local/include/X11/qt5/QtWebKitWidgets -isystem /usr/local/include/X11/qt5/QtWidgets -isystem /usr/local/include/X11/qt5/QtWebKit -isystem /usr/local/include/X11/qt5/QtGui -isystem /usr/local/include/X11/qt5/QtNetwork -isystem /usr/local/include/X11/qt5/QtCore -Ibuild -I/usr/X11R6/include -isystem /usr/local/include -I/usr/local/lib/qt5/mkspecs/openbsd-g++ -o build/BlockUrl.o BlockUrl.cpp
cc1plus: error: one or more PCH files were found, but they were invalid
cc1plus: error: use -Winvalid-pch for more information
cc1plus: fatal error: build/webkit_server: No such file or directory

I didn't find any relevant-looking recent changes, did anyone spot
anything going in that might be implicated?