Bug#914585: mathgl: qtwebengine5-dev is not available on all architectures

[Adrian Bunk]

Source: mathgl
Version: 2.4.2.1-2
Severity: serious
Tags: ftbfs

qtwebengine5-dev is only available on the following architectures:
  amd64 arm64 armhf i386 mipsel

mathgl tries qtwebengine5-dev first with a fallback to libqt5webkit5-dev,
which gives two possible solutions:

1. If using qtwebengine5-dev instead of libqt5webkit5-dev does
   not bring any significant improvements, it would be easiest
   to go back to using libqt5webkit5-dev on all architectures.

OR

2. Use qtwebengine5-dev where available and libqt5webkit5-dev
   everywhere else, by changing the build dependency to
     qtwebengine5-dev [amd64 arm64 armhf i386 mipsel]