Remove the use of BUILT_SOURCES

Fixes https://github.com/google/protobuf/issues/3912

[BUILT_SOURCES](https://www.gnu.org/software/automake/manual/html_node/Sources.html)
is used with [Automake's automatic dependency tracking](https://www.gnu.org/software/automake/manual/html_node/Dependencies.html#Dependencies)
but doesn't work well in protobuf when cross-compiling. Their presence
causes maek to always generate them even when they are not requested
and as a result causes cross-compilation to fail because the built protoc
cannot be used to generate unittest protos (see: https://github.com/google/protobuf/issues/3912).

Removing it with explicit dependencies fixes the build issue when running
'make', 'make all', 'make install' at cross-compilation. It doesn't affect
'make protoc' because BUILT_SOURCES only works for the implicit targets
'all', 'check' or 'install'.

src/Makefile.am

@@ -722,8 +722,6 @@ protoc_outputs =                                                  \
   google/protobuf/util/message_differencer_unittest.pb.cc         \
   google/protobuf/util/message_differencer_unittest.pb.h
 
-BUILT_SOURCES = $(protoc_outputs) google/protobuf/compiler/js/well_known_types_embed.cc
-
 if USE_EXTERNAL_PROTOC
 
 unittest_proto_middleman: $(protoc_inputs)
@@ -846,9 +844,8 @@ protobuf_test_SOURCES =                                        \
   google/protobuf/util/time_util_test.cc                       \
   google/protobuf/util/type_resolver_util_test.cc              \
   $(COMMON_TEST_SOURCES)
-
-
 nodist_protobuf_test_SOURCES = $(protoc_outputs)
+$(am_protobuf_test_OBJECTS): unittest_proto_middleman
 
 # Run cpp_unittest again with PROTOBUF_TEST_NO_DESCRIPTORS defined.
 protobuf_lazy_descriptor_test_LDADD = $(PTHREAD_LIBS) libprotobuf.la \
@@ -864,6 +861,7 @@ protobuf_lazy_descriptor_test_SOURCES =                        \
   google/protobuf/compiler/cpp/cpp_unittest.cc                 \
   $(COMMON_TEST_SOURCES)
 nodist_protobuf_lazy_descriptor_test_SOURCES = $(protoc_outputs)
+$(am_protobuf_lazy_descriptor_test_OBJECTS): unittest_proto_middleman
 
 COMMON_LITE_TEST_SOURCES =                                             \
   google/protobuf/arena_test_util.cc                                   \
@@ -888,6 +886,7 @@ protobuf_lite_test_SOURCES =                                           \
   google/protobuf/lite_unittest.cc                                     \
   $(COMMON_LITE_TEST_SOURCES)
 nodist_protobuf_lite_test_SOURCES = $(protoc_lite_outputs)
+$(am_protobuf_lite_test_OBJECTS): unittest_proto_middleman
 
 # lite_arena_unittest depends on gtest because teboring@ found that without
 # gtest when building the test internally our memory sanitizer doesn't detect
@@ -903,6 +902,7 @@ protobuf_lite_arena_test_SOURCES =       \
   google/protobuf/lite_arena_unittest.cc \
   $(COMMON_LITE_TEST_SOURCES)
 nodist_protobuf_lite_arena_test_SOURCES = $(protoc_lite_outputs)
+$(am_protobuf_lite_arena_test_OBJECTS): unittest_proto_middleman
 
 # Test plugin binary.
 test_plugin_LDADD = $(PTHREAD_LIBS) libprotobuf.la libprotoc.la \