ninja error building webkit2-gtk3 on rpi3

[bob prohaska]

Ports are at 484411, system is at 1300003. Attempts to compile webkit2-gtk3
stops with

....
 -c DerivedSources/JavaScriptCore/unified-sources/UnifiedSource86.cpp
ninja: build stopped: subcommand failed.
===> Compilation failed unexpectedly.
Try to set MAKE_JOBS_UNSAFE=yes and rebuild before reporting the failure to
the maintainer.
*** Error code 1

Stop.
make: stopped in /usr/ports/www/webkit2-gtk3

I can't find a more detailed statement of what went wrong anywhere in the
make output. 

Thanks for reading, any suggestions appreciated.

bob prohaska