cpp_stl/{clang3.4,gcc4.8}: reduce Boost build output

`-d0` means "Suppress all informational messages", see
https://www.boost.org/doc/libs/1_81_0/tools/build/doc/html/index.html#bbv2.overview.invocation.options

Without this option, the build output is flooded with 15k `common.copy`
messages like the following:

```
common.copy /usr/local/include/boost/wave.hpp
common.copy /usr/local/include/boost/weak_ptr.hpp
common.copy /usr/local/include/boost/version.hpp
common.copy /usr/local/include/boost/visit_each.hpp
(...)
```

... which is not very useful.

src/cpp_stl/clang3.4-linux-x86_64/Dockerfile

@@ -55,7 +55,7 @@ RUN curl -fsSLO https://archives.boost.io/release/1.81.0/source/boost_1_81_0.tar
 	rm -f boost_1_81_0.tar.bz2 && \
 	cd boost_1_81_0 && \
 	./bootstrap.sh --with-libraries=test && \
-	./b2 --with-test variant=debug link=shared install && \
+	./b2 -d0 --with-test variant=debug link=shared install && \
 	cd .. && \
 	rm -rf boost_1_81_0
 

src/cpp_stl/gcc4.8-linux-x86_64/Dockerfile

@@ -52,7 +52,7 @@ RUN curl -fsSLO https://archives.boost.io/release/1.81.0/source/boost_1_81_0.tar
 	rm -f boost_1_81_0.tar.bz2 && \
 	cd boost_1_81_0 && \
 	./bootstrap.sh --with-libraries=test && \
-	./b2 --with-test variant=debug link=shared install && \
+	./b2 -d0 --with-test variant=debug link=shared install && \
 	cd .. && \
 	rm -rf boost_1_81_0