[cmake] pass compiler to & use ccache for FFmpeg

[Rechi]

Motivation and Context

use ccache on FreeBSD jenkins builder for FFmpeg

How Has This Been Tested?

tested on linux without setting ENABLE_CCACHE and with setting ENABLE_CCACHE to ON & OFF
also tested with setting clang as compiler

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[Rechi]

@wsnipex comment added

[wsnipex]

lgtm