Not all environments have 'diff' installed. Most notably CentOS 8
does not have diff out-of-the-box. Thus users running 'cmake .' and
'make' would fail to build MariaDB, and they would think the error
was in ABI incompatibilities due to the error message emitted by CMake
when in reality simply 'diff' was missing.

This fixes it and makes the developer experience better by simply
skipping the diffing if 'diff' is not found.

Closes #1846