drop -march=native from CXXFLAGS

TL;DR: never ever use -march=native in CXXFLAGS for distro builds.

-march=native optimizes the build for the specific hardware it is
built on; this means that, for example, newer hardware features such
as AVX on x86_64, may be used, rendering the resulting binaries
completely unusable on hardware without the build-time bits used.
Because of this, -march=native is a giant no-no in distribution builds,
as you never want to optimize something based on the specific buildd
used, but rather make sure it will work on the architecture in general,
in a way compatible with the architecture baseline.

debian/changelog

@@ -1,5 +1,10 @@
 qt6-base (6.3.1+dfsg-4) UNRELEASED; urgency=medium
 
+  [ Pino Toscano ]
+  * Drop -march=native from the CXXFLAGS, as it will optimize the build for the
+    hardware it is built on (i.e. a specific buildd for each architecture),
+    making the binaries unusable (SIGBUS, "Illegal instruction") on hardware
+    without the build-time features.
 
  -- Debian Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>  Sat, 16 Jul 2022 13:12:46 +0200
 

debian/rules

@@ -19,9 +19,6 @@ else
         extra_cmake_args += -DFEATURE_sctp=OFF
 endif
 
-# Prevent "Error: selected processor does not support `yield' in ARM mode"
-export DEB_CXXFLAGS_MAINT_APPEND = -march=native
-
 %:
 	dh $@ --with pkgkde_symbolshelper --buildsystem=cmake+ninja