I am unable to build dev-qt/qtmultimedia-5.3.1 on a fresh installation because of the build failures attached to this bug. It appears this library depends on dev-qt/qtopengl-5. Building qtmultimedia after manually installing said library is successful on my ~amd64 machine.
Created attachment 381850 [details]
dev-qt/qtmultimedia-5.3.1 build log
The problem is that both QT_OPENGL and QT_NO_OPENGL are defined (the former presumably because of qtgui[opengl], the latter because multimediawidgets.pro adds it to DEFINES when qtopengl is not installed), therefore QtCore/qconfig.h #undef's QT_NO_OPENGL and the code tries to include some qtopengl headers.
Should be "fixed" with https://git.overlays.gentoo.org/gitweb/?p=proj/qt.git;a=commit;h=ea04ace482dc579e72e7aa74ecb412390fe19911
Although technically depending on qtgui[-opengl] with USE=-opengl would not be necessary if qtmultimedia used a different macro to disable opengl support.
Thanks for reporting.