try_compile: Add policy CMP0067 to honor language standards

Projects use `try_compile` to check if they will be able to compile some
particular source code.  When a language standard variable like
`CMAKE_CXX_STANDARD` is set, then the project intends to compile source
code using a compiler mode for that standard.  Therefore it makes sense
for `try_compile` to use that standard in the test project too.

Unfortunately this was not done when support for the
`CMAKE_CXX_STANDARD` variable was first implemented.  Add a policy to
introduce the improved behavior in a compatible way.

Closes: #16456

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