dropwatch: fix avr32 build failure

On avr32, dropwatch needs libiberty to avoid build failures such as

  http://autobuild.buildroot.net/results/cd5/cd5e22fb5c9b0fc5d396bc85a5e253a1a65054da/



Also test built on ARM.

[Peter: pass TARGET_LDFLAGS as well, append -lintl in gettext case]
Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>