awk: Prepare for Gawk 5.0.

* src/Makefile.am: Use pkg_namespace (instead of namespace).
* src/mkerrnos.awk: Likewise.
* lang/cl/mkerrcodes.awk: Don't escape # in regexp.
* src/mkerrcodes.awk, src/mkerrcodes1.awk, src/mkerrcodes2.awk: Ditto.

In Gawk 5.0, regexp routines are replaced by Gnulib implementation,
which only allows escaping specific characters.

• GnuPG-bug-id: T4459
• Reported-by: Marius Schamschula
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>