See individual commit messages. These are improvements in preparation of
the switch to Cirrus CI. (Maybe I'll just open a PR on top of this one.)

The first commit made the difference between successful build
https://cirrus-ci.com/task/6740575057608704 and unsuccessful build
https://cirrus-ci.com/task/4909571074424832.

I've tested the second commit without cross-compilation and with
cross-compilation for android (#621 (comment))

When working on the autoconf stuff, I noticed two things that I just
want to write down here:

    At some point we should update build-aux/m4/ax_prog_cc_for_build.m4.
This is outdated, and there have been a lot of fixes But the latest
version is broken, so now is probably not the time.
    The latest autoconf 2.70 deprecates AC_PROG_CC_C89. It's not needed
anymore because AC_PROG_CC cares about testing for version support. This
makes autoconf 2.70 output a warning that we should probably just
ignore. We don't want to force users onto 2.70...

./autogen.sh
./configure
make