Snappy is written in C++ and as such needs to link against libstdc++.  When
linking statically, this means that the compile test cannot succeed unless
performed with a C++ compiler.  Do so if link_language is set to C++; if it
is C, the test will usually fail and snappy will be disabled.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>