Packages that don't declare USE_TOOLS+=perl and whose configure script
invokes perl produce a warning.
Usually warnings are ignored, but they can also be configured as errors,
for example during a strict bulk build. In this situation it is
necessary to override the default behavior of the perl tool to fail
silently. Up to now, defining both TOOLS_BROKEN+=perl and
TOOLS_FAIL+=perl produced a duplicate make target.
To handle this situation, let TOOLS_BROKEN+=perl take precedence over
TOOLS_FAIL+=perl. This is much easier than finding out in each case how
to disable the perl check in the configure script, which is most often
done by adding any of the following to CONFIGURE_ENV: PERL=#none,
ac_cv_prog_PERL=#none, ac_cv_path_PERL=#none.
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