Trac #33153: opensuse-tumbleweed: sagelib build fails

As reported in https://groups.google.com/g/sage-
devel/c/81unMG60MyU/m/JdMKMFOSBQAJ

Workaround: `export CFLAGS="${CFLAGS} -Wno-error=return-type"`
(https://groups.google.com/g/sage-devel/c/81unMG60MyU/m/TfBT8KOsBQAJ)

This is caused by strict compiler flags leaking from the opensuse
python3 package into user packages via sysconfig/distutils.
Same mechanism as the failures with misconfigured python3 in #31132
(homebrew), #33078 (cygwin), #32531 (pyenv)

Upstream issue:
- https://bugzilla.opensuse.org/show_bug.cgi?id=1194605

URL: https://trac.sagemath.org/33153
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Michael Orlitzky

src/sage/libs/linbox/fflas.pxd

@@ -1,4 +1,12 @@
-# distutils: extra_compile_args = FFLASFFPACK_CFLAGS
+# Trac #33153: fflas-ffpack-2.4.3 is missing a return value in one of
+# its functions and runs afoul of -Werror=return-type. Compounding the
+# problem on openSUSE tumbleweed, the CFLAGS in python's sysconfig
+# contain -Werror=return-type and wind up being used to compile this
+# extension. To avoid a compilation failure on that platform, we
+# temporarily append "-Wno-error=return-type" to those flags.
+#
+# distutils: extra_compile_args = FFLASFFPACK_CFLAGS -Wno-error=return-type
+#
 # distutils: include_dirs = FFLASFFPACK_INCDIR
 # distutils: libraries = FFLASFFPACK_LIBRARIES
 # distutils: library_dirs = FFLASFFPACK_LIBDIR