tests: avoid one tricky use of "make -e"

tests: avoid one tricky use of "make -e"

Which was causing a spurious failure on FreeBSD.  Not particularly
surprising, given how brittle "make -e" is in general ...

* t/cxx-lt-demo.sh: Instead of forcing $(CC) to be 'false' by
exporting "CC=false" in the environment and then passing the '-e'
option to make, do so by passing "CC=false" on the make command
line, both directly and using AM_MAKEFLAGS.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>