Embedded vt as a subdirectory exports ccache usage to enclosing project

[PhilMiller]

Describe the bug

An application that embeds vt and refers to it via add_subdirectory finds that it gets ccache picked up automatically, regardless of whether the person building the application wanted it, or whether that application has its own configuration mechanism to choose to use ccache. Vt should definitely not be doing this in the embedded case.

Offending CMake code is here:
https://github.com/DARMA-tasking/vt/blob/develop/CMakeLists.txt#L28

This corresponds to DARMA-tasking/magistrate#302

To Reproduce
Steps to reproduce the behavior:

1. Example/test/snippet of code that fails
2. Compiler, platform, libraries
3. Run command: number of processors, threading options, etc.
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-4.0, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
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[nlslatt]

@PhilMiller What were you envisioning as a solution here? Make using ccache always opt-in or have differing behavior for embedded in application vs. standalone builds?

[PhilMiller]

I don't have a particular solution in mind.

The simplest approach, I suppose, is to remove the integrated ccache usage, and just let that be something that someone (or some script) building vt specify by the usual CMake means