bug#47569: ‘qt-build-system’ retains too many references via wrappers

["Ludovic Courtès" <ludo@gnu.org>]

I just noticed this:

--8<---------------cut here---------------start------------->8---
$ guix size ktouch | tail -1
total: 1752.4 MiB
--8<---------------cut here---------------end--------------->8---

Sounds a lot for a touch typing tutor.

Turns out ktouch references all its build-time dependencies: gcc,
glibc:static, binutils, findutils, everything.  It comes from the
wrapper:

--8<---------------cut here---------------start------------->8---
$ grep -r s3dcqzwqaakv1yx37by9chksdbkgih17  /gnu/store/w8i63ghrq64r7f1nxzw50b0j32f2n8v1-ktouch-20.12.1
/gnu/store/w8i63ghrq64r7f1nxzw50b0j32f2n8v1-ktouch-20.12.1/bin/ktouch:export XDG_DATA_DIRS="/gnu/store/w8i63ghrq64r7f1nxzw50b0j32f2n8v1-ktouch-20.12.1/share:/gnu/store/ah4az84vjkrssdws7ydvdk1f1mcs70i6-cmake-minimal-3.16.5/share:/gnu/store/3x63j4ndhbzkjwr6mhnp4h86r550gs7q-extra-cmake-modules-5.70.0/share:/gnu/store/qcqghpj3550p94pyqwqmwyd3nqcwflpq-kdoctools-5.70.0/share:/gnu/store/krpyb0zi700dcrg9cc8932w4v0qivdg9-pkg-config-0.29.2/share:/gnu/store/v6f44zccwh9z5zk3pjlywjybbi8n2hjh-tar-1.32/share:/gnu/store/ncydgq2znms5n1d2k5yqshhf58nsixwv-gzip-1.10/share:/gnu/store/i8h2pcxqdq07ijm3ibkka8f4smn1w48v-bzip2-1.0.8/share:/gnu/store/9860f1abqj8wjjnwl8a9v54pdcc3bhgf-xz-5.2.4/share:/gnu/store/60g7r3l01fd7c58yjbm6krgcwj1jkpwg-file-5.38/share:/gnu/store/n4n560pfvvw50a9369axw5vj5rrqfj1n-diffutils-3.7/share:/gnu/store/cd5qf3kcnlq35p9k392pjdpdzpsnds70-patch-2.7.6/share:/gnu/store/hic7snhayfl7m6cpfqqr73nmm19bpqkg-findutils-4.7.0/share:/gnu/store/swqdvwri9dbv6zssg6v0by7l05hd6wxp-gawk-5.0.1/share:/gnu/store/ishk7fswcs4gkwcp8mh788z4mvvl9bxh-sed-4.8/share:/gnu/store/bhs4rj58v8j1narb2454raan2ps38xd8-grep-3.4/share:/gnu/store/57xj5gcy1jbl9ai2lnrqnpr0dald9i65-coreutils-8.32/share:/gnu/store/hm40bxnv8jxmbc1lpb7zfimii4xm9m81-make-4.3/share:/gnu/store/pwcp239kjf7lnj5i4lkdzcfcxwcfyk72-bash-minimal-5.0.16/share:/gnu/store/m1z7cdbqsqyp9xnjw5cvlb4a7gkcg3m4-binutils-2.34/share:/gnu/store/rn75fm7adgx3pw5j8pg3bczfqq1y17lk-gcc-7.5.0/share:/gnu/store/fa6wj5bxkj5ll1d7292a70knmyl7a0cr-glibc-2.31/share:/gnu/store/s3dcqzwqaakv1yx37by9chksdbkgih17-glibc-2.31-static/share:/gnu/store/nnri2x0mdfm2y2ci0p5jhlf6xkdsf6i6-kcmutils-5.70.0/share:/gnu/store/ybh1br5ghs1brnwvacfj04jx3203m87p-kcompletion-5.70.0/share:/gnu/store/bnfcann370ad38g7v4pzgklmizk69f7j-kconfig-5.70.0/share:/gnu/store/4w9v0jnx74n542v11ikzxbf352xqrpwy-kconfigwidgets-5.70.0/share:/gnu/store/9nr52vkhj99g4mx1rr349wwsm2aflnv4-kcoreaddons-5.70.0/share:/gnu/store/9ln7cv5s813nf4ny5l56qadq0vnkx5g0-kdeclarative-5.70.0/share:/gnu/store/fg8npcm84k5bii5jpmdypqdwq10k5xrf-ki18n-5.70.0/share:/gnu/store/0b7xb35ihp5h5qzfcdymzi7yn5lngsgf-kiconthemes-5.70.0/share:/gnu/store/gdch1j5vjjzzx9qq29zca2w83h8nz9rj-kitemviews-5.70.0/share:/gnu/store/q8wrlw1g0m35syx5sr3hq44sx6fsxicj-kqtquickcharts-20.04.1/share:/gnu/store/xhcgpv14llkcdcdn39vrm12mbl4b4y07-ktextwidgets-5.70.0/share:/gnu/store/f4ibdx1ldiifdjbpyh75py4q8i24b05w-kwidgetsaddons-5.70.0/share:/gnu/store/x27qq3i8c3vifrj1q5y9p8840k5dvk40-kwindowsystem-5.70.0/share:/gnu/store/ybdzlpnswajsq1acp3l768h21bvzif16-kxmlgui-5.70.0/share:/gnu/store/3x6y8kpbwjmzpndjfhdbn654g7dm85cp-libxcb-1.14/share:/gnu/store/c7vrkq864hsxfvll74wlrxv71arkcd5q-libxkbfile-1.1.0/share:/gnu/store/sbf77k7xzwhkx4ygqjzgkviz9vy4qhf6-qtbase-5.15.2/share:/gnu/store/k6kk02q27nbymwajkvzhlil8c7sym74k-qtdeclarative-5.15.2/share:/gnu/store/hpirydj1mlmhj199z1ng3yn9sb0f1jc1-qtgraphicaleffects-5.15.2/share:/gnu/store/8165pcgy2d1l30vipkv5qz40nkg9qdp3-qtquickcontrols2-5.15.2/share:/gnu/store/63g70yiyvz08v16hdxsnj3c2pqdq9bm0-qtx11extras-5.15.2/share:/gnu/store/9hpw67pf9px9zx75nb8bm21pc0gqwyx9-qtxmlpatterns-5.15.2/share:/gnu/store/gfapkk5c6hvl1d94m4sqnhn7f9l5gqyh-linux-libre-headers-5.4.20/share:/gnu/store/lfizifxrrbmb9dd1wdm5yy7rlqp7mmfp-kservice-5.70.0/share:/gnu/store/yqaafdggah0hac4n6dkw69ib94v03w69-kcodecs-5.70.0/share:/gnu/store/1syzh2ky8qgxzg59g0g97pqn1gb6xm1r-kauth-5.70.0/share:/gnu/store/s7a55hh8gd8r9dr6m04sqxg5kgngnqcj-kpackage-5.70.0/share:/gnu/store/cizh7vg0w09izkv07pxdv8csir8p4sdd-python-3.8.2/share:/gnu/store/n9wx4dp68bfsw9bb1bwmklzl34ggrmgm-gettext-minimal-0.20.1/share:/gnu/store/rqsn0is9frgi7jz1633l4ipa07xfgsbx-sonnet-5.70.0/share:/gnu/store/6cdl970wcv4jhvpgbh8sdj54a5gwhmwj-libxdmcp-1.1.3/share:/gnu/store/h7sy4hr7arjknbyy1aq0xwv6fksnzw9n-libxau-1.0.9/share:/gnu/store/klk98l8czl05lvanrr7xl7a9hksac4ak-libpthread-stubs-0.4/share:/gnu/store/k30a2pgmc6b3ifmq3rn0dr5ng9gr8w81-which-2.21/share:/gnu/store/14qgm67plbh8d3m6lc87b9j6s9296vwn-mesa-20.2.4/share:/gnu/store/wxl57nkbqgamfp73b7v62kk3f1hiv0cz-xorgproto-2019.2/share:/gnu/store/qv7x7pz7gi724lkdz8049mjhy1ryg511-libxxf86vm-1.1.4/share:/gnu/store/jwga98k68l0h5c45jx7z4jdjzhfc34vm-libxshmfence-1.3/share:/gnu/store/1v6vap6jaf3hf4ypjx261k44xmc48drc-libxfixes-5.0.3/share:/gnu/store/b67b0a5vda7pv7yqyjjb9736s96hhybi-libxdamage-1.1.5/share:/gnu/store/rwkqxykm91a75w9afhb41saj0dmf30hw-libx11-1.6.12/share:/gnu/store/kx58xc3hxa9b0mp1hl7pzz43bl7gh18h-libdrm-2.4.103/share:/gnu/store/lc8irxnwazq340ygkbqh0ask446jzy9k-util-macros-1.19.2/share:/gnu/store/3rmazp46f6g8w9qs8n3w7qcg8hhs1lig-libxext-1.3.4/share"
--8<---------------cut here---------------end--------------->8---

This is bad for the closure size and probably not good for application
startup time either.

The problem seems that ‘variables-for-wrapping’ in qt-build-system.scm
is too naive: it collects in XDG_DATA_DIRS all the /share directories.
Conversely, ‘data-directories’ in glib-or-gtk-build-system.scm is a bit
more subtle, keeping only meaningful share/ directories.

I think qt-build-system.scm should copy that strategy.

Thoughts?

Ludo’.