Don't warn on missing clang-format/tidy when disabled

[xxxxxxjun]

Fixes #1158.

cmake_dependent_option forces ENABLE_FORMAT/ENABLE_LINTING to OFF
when the tool is missing, so the existing elseif(NOT CLANG_FORMAT_PATH)
can't tell an explicit OFF from a missing tool and warns in both cases.

Changes:

• Record whether the user set the option to OFF before
  cmake_dependent_option runs, and only warn when they didn't.
• Drop a redundant option(ENABLE_LINTING ...) that the following
  cmake_dependent_option immediately overrode.

Verified by configuring with the tool present and missing:

• missing, default: warns (unchanged)
• missing, OFF: no warning (the fix), including on reconfigure
• missing, ON: warns (unchanged)
• present: no warning (unchanged)

[coveralls]

coverage: 99.176%. remained the same — xxxxxxjun:fix/cmake-warning-on-disabled-option into uber:master