t/unit-tests: convert "clar-generate.awk" into a shell script

t/unit-tests: convert "clar-generate.awk" into a shell script

Convert "clar-generate.awk" into a shell script that invokes awk(1).
This allows us to avoid the shell redirect in the build system, which
may otherwise be a problem with build systems on platforms that use a
different shell.

While at it, wrap the overly long lines in the CMake build instructions.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>