We could add AC_PREFIX_PROGRAM(gawk) to configure.ac.
This would cause the configure script to search the
PATH for the current location of gawk, and then set
the prefix automatically to install xgawk in the same
place.
Personally, I think this is a dangerous idea, because
it makes the install location uncertain. On my linux
system, gawk is installed in /bin/gawk and in
/usr/bin/gawk, so the prefix could be / or /usr depending
on which instance of gawk is first in my PATH. So
I think this is too unpredictable, and we should
require the user to specify the installation directory.
No longer relevant.