ktwebkit 5.7.1 fails to compile on gentoo

localhost ~ # emerge --oneshot --quiet =dev-qt/qtwebkit-5.7.1
...

>>> Verifying ebuild manifests
>>> Emerging (1 of 1) dev-qt/qtwebkit-5.7.1::gentoo
>>> Failed to emerge dev-qt/qtwebkit-5.7.1, Log file:
>>> '/var/tmp/portage/dev-qt/qtwebkit-5.7.1/temp/build.log'

(error snippet)...
make[2]: Entering directory '/var/tmp/portage/dev-qt/qtwebkit-5.7.1/work/qtwebkit-opensource-src-5.7.1/Source/JavaScriptCore'ruby /var/tmp/portage/dev-qt/qtwebkit-5.7.1/work/qtwebkit-opensource-src-5.7.1/Source/JavaScriptCore/offlineasm/generate_offset_extractor.rb llint/LowLevelInterpreter.asm LLIntDesiredOffsets.h/usr/lib64/ruby/2.0.0/rubygems.rb:15:in `require': cannot load such file -- rubygems/compatibility (LoadError) from /usr/lib64/ruby/2.0.0/rubygems.rb:15:in `<top (required)>' from <internal:gem_prelude>:1:in `require' from <internal:gem_prelude>:1:in `<compiled>'make[2]: *** [Makefile.LLIntOffsetsExtractor:487: LLIntDesiredOffsets.h] Error 1make[2]: Leaving directory '/var/tmp/portage/dev-qt/qtwebkit-5.7.1/work/qtwebkit-opensource-src-5.7.1/Source/JavaScriptCore'make[1]: *** [Makefile.JavaScriptCore:42: sub-LLIntOffsetsExtractor-pro-make_first-ordered] Error 2make[1]: Leaving directory '/var/tmp/portage/dev-qt/qtwebkit-5.7.1/work/qtwebkit-opensource-src-5.7.1/Source/JavaScriptCore'make: *** [Makefile:93: sub-Source-JavaScriptCore-JavaScriptCore-pro-make_first-ordered] Error 2
* ERROR: dev-qt/qtwebkit-5.7.1::gentoo failed (compile phase): * emake failed *
...

localhost ~ # eselect ruby list
Available Ruby profiles:
[1] ruby20 *
[2] ruby21 (with Rubygems)
[3] ruby22 (with Rubygems)
[4] ruby23 (with Rubygems)

localhost ~ # eselect ruby set 4

Successfully switched to profile: ruby23

localhost ~ # emerge --oneshot --quiet =dev-qt/qtwebkit-5.7.1

>>> Verifying ebuild manifests
>>> Emerging (1 of 1) dev-qt/qtwebkit-5.7.1::gentoo
>>> Installing (1 of 1) dev-qt/qtwebkit-5.7.1::gentoo

!!! existing preserved libs found