Use AC_PROG_AWK and $AWK in DX_LINGUAS.

The awk script used internally by this macro does not work on traditional
awk, such as old Solaris /bin/awk.  Use $AWK to get a better awk.

To avoid similar problems in the future, arrange for tests to be run
with a busted "awk" command in PATH, so only $AWK will work at all.