build: add a SlackBuild under assets/slackbuild

Packages the application for Slackware, following SBo conventions with
two deliberate departures: the tag is _danix rather than _SBo and the
package type is txz rather than tgz, since this is not an SBo
submission. sbolint reports exactly those two as errors and nothing
else; the template comments it warned about are gone.

Written against the install layout the build really produces, checked by
staging it: one binary, one .desktop entry and one scalable icon, no
libraries and no man or info pages. The template's .la removal, man and
info compression and perllocal.pod cleanup would all act on nothing
here, so they are left out rather than carried along as dead code.
doinst.sh keeps only the desktop-database and icon-cache updates.

The download URL and checksum are verified rather than assumed: sbodl
fetches the tarball and reports "md5sum matches OK".

Adds QTMAILDIR_BUILD_TESTS, defaulting to ON so the ordinary build is
unchanged. A packaging build has no use for the test binaries, and
building them pulls in Qt6::Test to produce nothing that ships. Note the
0.4.0 tarball predates this option, so with that source the flag is
accepted but does nothing; the README says so.

notmuch is the only dependency outside Slackware. Qt6 including
WebEngine, gmime and cmake are all stock, which is what REQUIRES
reflects.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/README.md b/README.md
index 293d733..8a994d3 100644
--- a/README.md
+++ b/README.md
@@ -60,6 +60,29 @@ A single test binary, for a tighter loop:
 ./build/tests/test_keymap
 ```
 
+Packaging builds do not need the tests, and building them pulls in `Qt6::Test`
+to produce nothing that ships:
+
+```bash
+cmake -S . -B build -DQTMAILDIR_BUILD_TESTS=OFF
+```
+
+### Slackware package
+
+`assets/slackbuild/` holds a SlackBuild. It follows SBo conventions, with two
+deliberate departures: the tag is `_danix` rather than `_SBo`, and the package
+type is `txz` rather than `tgz`, since this is not an SBo submission. `sbolint`
+reports exactly those two as errors and nothing else.
+
+```bash
+cd assets/slackbuild
+# fetch the source tarball named in qtmaildir.info next to the script
+sudo ./qtmaildir.SlackBuild
+```
+
+`notmuch` is the only dependency outside Slackware itself; Qt6 (WebEngine
+included), gmime and cmake are all stock.
+
 ## Configuration
 
 `~/.config/qtmaildir/qtmaildir.conf`, INI format.