When the build directory isn't the same as the source directory the
google/protobuf/compiler/js directory doesn't exist.  Make sure that it does
before trying to stick a file in there.

Sent upstream as https://github.com/google/protobuf/pull/3205

diff --git a/src/Makefile.am b/src/Makefile.am
index bfb875ac..dd041cb6 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -519,7 +519,7 @@ js_well_known_types_sources =                                  \
 	google/protobuf/compiler/js/well_known_types/timestamp.js
 # We have to cd to $(srcdir) so that out-of-tree builds work properly.
 google/protobuf/compiler/js/well_known_types_embed.cc: js_embed$(EXEEXT) $(js_well_known_types_sources)
-	oldpwd=`pwd` && cd $(srcdir) && \
+	oldpwd=`pwd` && cd $(srcdir) && mkdir -p $$(dirname $$oldpwd/$@) && \
 	$$oldpwd/js_embed$(EXEEXT) $(js_well_known_types_sources) > $$oldpwd/$@
 
 # Tests ==============================================================