Instead of cleaning, I can recommend using
./b2 --build-dir=/path/to/temp/combustible
for assembly. This puts all intermediate / temporary files in a separate place. I always put it in tmpfs ("ram ram"), so it went after a reboot anyway. But you can trivially just remove this build-dir as soon as you lose interest.
The approach works on all platforms. You can store intermediate products until all of your builds are completed to maximize the use of shared intermediate files.
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