Makefile: run 'rebar3 clean' upon make clean

Otherwise the diameter files generated during compile in include/ and
src/ are not removed.

Change-Id: I771860f965499bb754c13385819845ef23f33553
---
M Makefile
1 file changed, 20 insertions(+), 0 deletions(-)

diff --git a/Makefile b/Makefile
index b2fe66f..0470458 100644
--- a/Makefile
+++ b/Makefile
@@ -14,4 +14,12 @@
 	rebar3 eunit
 
 clean:
+# Avoid running rebar3 clean if _build doesn't exist, since it would try
+# to fetch deps from the Internet and that may not be avaialble when in
+# OBS build.
+ifneq ("$(wildcard _build/.*)", "")
+	rebar3 clean
 	rm -rf _build/
+else
+	rm -f src/diameter_*.erl
+endif

