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'.