[CMAKE] Make the source control tools inclusion an option

Summary:
And disable it by default. The check-source-control-tools would fail
on systems where bash in not installed. With this diff cmake errors
out with a meaningful message if unavailable.

Test Plan:

sed -i 's/bash/trash/g' ../contrib/source-control-tools/CMakeLists.txt

Should work:

cmake -GNinja ..

Should fail:

rm -rf * && cmake -GNinja .. -DBUILD_SOURCE_CONTROL_TOOLS=ON

sed -i 's/trash/bash/g' ../contrib/source-control-tools/CMakeLists.txt

Should Work:

rm -rf * && cmake -GNinja .. -DBUILD_SOURCE_CONTROL_TOOLS=ON
ninja check-source-control-tools

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4645