For large projects, the default bzip2 compression of gcov data is
prohibitively expensively (could take several hours). Introduce options
that allow the consumer to choose between file size and compression time.
Add a new optional argument `TARBALL_COMPRESSION` for the macro
`ctest_coverage_collect_gcov`. This option accepts the values `GZIP`,
`BZIP2`, `XZ`, `FROM_EXT`, or an expression that evaluates to `FALSE`.
The default value is `BZIP2` to preserve prior behavior.
Fixes: #20593
40ca6ef125