app-admin/syslog-ng-4.9.0: fix build on non-glibc systems

Unconditionally disable stackdump support, which requires libunwind and
glibc support (dladdr()).  Upstream suggests to disable the feature by
default.  Perhaps a next revision/version can have a USE-flag for this
if deemed a useful feature (seems more like a development aid to me).

Closes: https://bugs.gentoo.org/963387
Signed-off-by: Fabian Groffen <grobian@gentoo.org>

app-admin/syslog-ng/syslog-ng-4.9.0.ebuild

@@ -123,6 +123,7 @@ src_configure() {
 		--disable-java
 		--disable-java-modules
 		--disable-riemann
+		--disable-stackdump  # 963387
 		--enable-ipv6
 		--enable-manpages
 		--localstatedir=/var/lib/syslog-ng