DX_GNULIB_SYMFILES: Don't use "awk -f -".

DX_GNULIB_SYMFILES: Don't use "awk -f -".

Even though a filename of "-" is specified by POSIX to read the program
from standard input and works with almost every awk implementation out
there, it seems it does not work on AIX 7.2 awk (which tries to read the
program from a file named "-" instead).  How wonderful.

In this case, it is easy enough to use a quoted string for the program
instead of a here-document.