We keep revisiting GMock. This time around dbus-cpp. It fails to cross
build. When it builds GMock, it detects the C compiler as /usr/bin/cc
and figures that it doesn't work. This happens e.g. when compiling for
arm64 on amd64, because both architectures have architecture-dependent
compiler flags and while CFLAGS is forwarded via the environment, the C
compiler is not, so the amd64 compiler is unhappy about the arm64 flags.

So we actually need to forward the C compiler as well (in addition to
the C++ compiler we are already forwarding). Please find another patch
attached.

Helmut