build: Set SHELL make variable to bash if possible
On Debian/Ubuntu systems, /bin/sh is /bin/dash which doesn't understand
the -e option to the `echo' builtin. Thus the build output looks
something like the following:
Building netsniff-ng toolkit (0.5.8-rc5+) for x86_64-linux-gnu:
Building netsniff-ng:
-e CC dissector.c
-e CC dissector_eth.c
-e CC dissector_80211.c
...
Prevent this by setting SHELL to /bin/bash if it exists on the system,
otherwise stick to /bin/sh.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>