package: set CXX to false rather than "" if no C++ support is enabled

Some configure scripts seems to ignore CXX settings if it is set to
the empty string, and goes back to the default (<arch>-linux-g++),
so use false instead, as that will loudly break the build if the
C++ compiler is ever used.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>