ccache does not recompile a source file when the source file hasn't changed and the command line neither, but the used plugins (-fplugin=<path to shared object>) were updated. To fix this, ccache should include the timestamps/hashes of all plugins used on the command line.
This was first noticed by getting unexpected results with the evolving gcc plugins of the PaX Linux kernel patch (<http://www.grsecurity.net/~paxguy1/>).
Fixed in 6802a3f6ca005138969a8badfa91abae2184de9f.
Included in v3.1.7.