Make default Makefile target "all".

Change-Id: I6b020876586fdbcf2952e4fddee86353d62b9ad0
Reviewed-on: https://weave-review.googlesource.com/2432
Reviewed-by: Alex Vakulenko <avakulenko@google.com>
diff --git a/Makefile b/Makefile
index 6fd30f5..df27b58 100644
--- a/Makefile
+++ b/Makefile
@@ -104,4 +104,5 @@
 cleanall : clean clean-gtest clean-libevhtp
 
 .PHONY : clean cleanall all
+.DEFAULT_GOAL := all