Autoconf: Clean up usage of "particular programs"

[erlend-aasland]

In configure.ac, we check for particular programs, but we rarely use the resulting variables; for example, instead of using $SED to call a POSIX compliant sed, we simply use sed directly.

IMO, we should clean up usages of "particular programs" in configure.ac and Makefile.pre.in, and consistently use the Autoconf variables.

See also:

• Improve and simplify configure.ac checks #89886 (comment)
• 'install -d' fails on BSDI systems. #34152
• Unable to build Python out-of-tree when source tree is readonly. #60023