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>