Add missing CFLAGS, CPPFLAGS and LDFLAGS for some examples/tests

[wiene]

This patch ensures that CFLAGS, CPPFLAGS and LDFLAGS settings are
respected when compiling/linking (lib)sotest, pivot_root and userns.

I was kindly made aware of this issue by the blhc tool.

[reidpr]

Thanks Peter!

I'd also be open to a PR adding blhc to the test suite. Do you think that's a good idea?

[reidpr]

These are C programs, so do we really want $CPPFLAGS?

[wiene]

Since the C compiler uses the C preprocessor, I think that it makes sense to also include C preprocessor flags.

[reidpr]

Oops, I'm sorry. I thought CPPFLAGS was for C++.

[wiene]

I'd also be open to a PR adding blhc to the test suite. Do you think that's a good idea?

So far I have no experience with using blhc outside a Debian environment where it is part of the CI machinery. Citing blhc's README file (for version 0.09):

At the moment it works only on Debian and derivatives but it should be easily
extendable to other systems as well.

[...]

The build log must contain the following line which marks the beginning of the
real compile process (output of dpkg-buildpackage):

dpkg-buildpackage: ...

If it's not present no compiler commands are detected. In case you don't use
dpkp-buildpackage but still want to check a build log, adding it as first line
should work fine.

So in summary, I feel uncertain how easy or difficult it is to integrate it in the Charliecloud test suite.

[reidpr]

So in summary, I feel uncertain how easy or difficult it is to integrate it in the Charliecloud test suite.

OK. This part of the code doesn't change too much, and it sounds like you would catch it eventually in the Debian package, so that seems like a low priority to add it upstream?