bootstrap: use a decent awk if /bin/awk is hopeless.

bootstrap: use a decent awk if /bin/awk is hopeless.

Bootstrap used to fail without an explicit AWK environment
setting on machines with both nawk and awk, but where awk is a
museum piece.
* gl/build-aux/inline-source (AWK): Remove initial setting.
(require_awk): Search PATH for a gawk, mawk or nawk before
settling on awk, just like configure does.
(func_include): Use it.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>