Bug#900554: gcc-8: don't do a full build for binary-indep builds

[Emilio Pozuelo Monfort <pochu@debian.org>]

Source: gcc-8
Version: 8.1.0-5
Severity: normal

Hi,

gcc-8 arch:all builds take up to 20h because they do a full build, only
to ship the arch-indep packages:

 gcc-8-locales deb devel optional arch=all
 gcc-8-source deb devel optional arch=all
 libgccjit-8-doc deb doc optional arch=all
 libstdc++-8-doc deb doc optional arch=all

It doesn't look like any of those would need to build all of GCC, so it
should be possible to split the -indep/-arch targets so that a binary-indep
build just does the minimum stuff it needs to build the above packages.

Cheers,
Emilio