externalsrc: do not call make clean for recipes with CLEANBROKEN = "1" set

ERROR: distrho-ports-0.0.0+git999-r0 do_buildclean: oe_runmake failed
ERROR: distrho-ports-0.0.0+git999-r0 do_buildclean: Function failed: do_buildclean
ERROR: Logfile of failure stored in: <...>/temp/log.do_buildclean.17285 Log data follows:

| DEBUG: Executing shell function do_buildclean
| NOTE: make clean
| make clean -C libs/drowaudio
| make[1]: Entering directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio'
| make clean -C build-drowaudio
| make[2]: Entering directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio/build-drowaudio'
| make[2]: *** No rule to make target 'clean'.  Stop.
| make[2]: Leaving directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio/build-drowaudio'
| make[1]: *** [Makefile:7: clean] Error 2
| make[1]: Leaving directory '/home/a.mueller/data/oe-core/workspace/sources/distrho-ports/libs/drowaudio'
| make: *** [Makefile:73: clean] Error 2
| ERROR: oe_runmake failed

---
 meta/classes/externalsrc.bbclass | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

index ac1b904e72..3f1d39689b 100644
@@ -173,7 +173,9 @@ do_buildclean[doc] = "Call 'make clean' or equivalent in ${B}"
 externalsrc_do_buildclean() {
 	if [ -e Makefile -o -e makefile -o -e GNUmakefile ]; then
 		rm -f ${@' '.join([x.split(':')[0] for x in (d.getVar('EXTERNALSRC_SYMLINKS') or '').split()])}
-		oe_runmake clean || die "make failed"
+		if [ "${CLEANBROKEN}" != "1" ]; then
+			oe_runmake clean || die "make failed"
+		fi
 	else
 		bbnote "nothing to do - no makefile found"
 	fi

2.14.3