cmake: use SH_EXE to execute clar scripts

cmake: use SH_EXE to execute clar scripts

In 30bf9f0aaa (cmake: set up proper dependencies for generated clar
headers, 2024-10-21), we have deduplicated the logic to generate our
clar headers by reusing the same scripts that our Makefile does. Despite
the deduplication, this refactoring also made us rebuild the headers in
case the source files change, which didn't happen previously.

The commit also introduced an issue though: we execute the scripts
directly, so when the host does not have "/bin/sh" available they will
fail. This is for example the case on Windows when importing the CMake
project into Microsoft Visual Studio.

Address the issue by invoking the scripts with `SH_EXE`, which contains
the discovered path of the shell interpreter.

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>