cygwin provides the header file but the lib file needs
to be added manually. A generic fix is to check if
we can compile and link a program that uses gettext()
and if it fails fall back to NO_NLS.

International users of cygwin may have to specify
HOST_LOADLIBES := "-lintl" on the make command line.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>