Fix building with gcc (#2747)
Fix building with gcc (#2747)

--config=libc++ is not compatible with gcc, since gcc does not
support -stdlib option. Therefore, disable usage of libc++ when
gcc is used to compile proxy. With this patch, the following
begins to work:

    make CC=/usr/bin/gcc CXX=/usr/bin/g++

Signed-off-by: Ilya Leoshkevich <425ffc1422dc4f32528bd9fd5af355fdb5c96192@linux.ibm.com>
istio/proxy,istio/proxy,istio/proxy,istio/proxy