`clang/tools/scan-build` is implemented in `perl`. However given `perl`
is not mentioned as a required dependency in `GettingStarted.rst` we
should make this optional.
This adds a `find_package(Perl)` check to cmake and disables the
`scan-build` tests when no perl executable is found.
Ideally we would also check if dependent perl modules like `Hash::Util`
are present on the system, but I don't see any pre-existing cmake macros
to easily test this. So for now I go with a plain check for the `perl`
package, at least this allows to use `cmake
-DCMAKE_DISABLE_FIND_PACKAGE_Perl=ON` to manually disable `perl` and the
tests.
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