Makefilegen: adopt for the Debian packaging

The Debian and Ubuntu packages install the
- headers into /usr/include/titan
- libraries into /usr/lib/titan

So the generated Makefile tries to detect the presence of these
directories if the TTCN3_DIR is not set, and adjust the CPPFLAGS and the
linker command

Signed-off-by: Gabor Szalai <gabor.szalai@ericsson.com>