Buildsystem wants CXX to be defined as $(CXX).
Otherwise it fallbacks to default value, which does not contain
"-std=c++11" statement.
Other changes (like CC=$(CC) and CFLAGS=$(CFLAGS) instead of CXX ones)
is not directly fix the issue, but fix cases, where CFLAGS (and CC)
differs from CXX* ones, so it could lead to similar errors in the future.