FindMySQL.cmake: Do not call try_run when cross-compiling

Thomas Ivesrequested to merge
fix/try-run-crosscompile into main

try_run produces an AUTHOR_WARNING if we run it when cross-compiling without CMAKE_CROSSCOMPILING_EMULATOR set suggesting we should avoid this.

This is a copy of TangoDatabase!73 (merged) to keep each copy of FindMySQL.cmake inline.

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