Fixed build error related to zlib on systems with older make versions (regression in ccache 3.2.3).
Made conversion-to-bool explicit to avoid build warnings (and potential runtime errors) on legacy systems.
Improved signal handling: Kill compiler on SIGTERM; wait for compiler to exit before exiting; die appropriately.
Minor fixes related to Windows support.
The correct compression level is now used if compression is requested.
Fixed a bug where cache cleanup could be run too early for caches larger than 64 GiB on 32-bit systems.
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