Perhaps this is a non-issue, and only gnu-make is supported
for this bootstrapping stage (and non-gnu makes are only supported
after "make dist"). Still, worth reporting.
Indeed, we need to distinguish the two uses: Building from a tarball,
and building from a git checkout.
Now, for building from a git checkout, typically more
tools are needed, usually listed in the file 'HACKING'. To me,
the priorities are as follows:
1. GNU make must be supported.
2. The preinstalled 'make' of various operating systems should
be supported, if possible.
3. Other 'make' implementations should be supported, if possible.
The 'bmake' here is BSD-make from the stock Debian 'bmake' package,
but the same happens on FreeBSD.
'bmake' is in category 3. Whereas the Makefile rule that rebuilds
iconv_open-aix.h has been written in such a way that it works with
OpenBSD 'make', which is in category 2.
I won't remove support for a 'make' in category 2, to satisfy one
in category 3.
In other words, 'bmake' is just too dumb to be supported as a build
tool in HACKING.