spawn+gcf2: improve diagnostics for build failures

I'm not sure why, but I noticed the one of my latest restarts of
public-inbox-httpd wasn't loading the Inline::C .so for Gcf2 nor
Spawn.  I also can't reproduce the problem as both .so files are
loaded fine on a restart with zero config changes.

In any case, some extra, automatic diagnostics for build errors
won't hurt, as no extra noise is introduced for successful builds.
This will also make future development of C code more convenient,
hopefully.

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