richliu's blog

If CPU number is too much, compile qtwebengine will fail, seems ninja compile issues.

it better to reduce CPU number under 8, batter number is 4 or 6.

just put MAKEOPTS=”-j 4″ on your /etc/portage/make.conf

It can be roll back after compile qtwebengine.

modify package.env can support special MAKEOPTS command on some packages.
ex:

echo "dev-qt/qtwebengine lowcpu.conf" | sudo tee -a /etc/portage/package.env
mkdir -p /etc/portage/env
echo "MAKEOPTS=\"-j4\"" | sudo tee /etc/portage/env/lowcpu.conf

ref.
[SOLVED] emerging qtwebengine-5.7.1
Memory problems on QtWebEngine build