[ccache] Build problem of qemu>=2.0 when using ccache.

[marcin kowalski]

Hi all, i (and another user) have been experiencing odd issues when 
building qemu-2.0 or 2.1 with ccache enabled. Systems in question are 
gentoo, 64bit installations with differing gcc versions.

Basically, it seems that build directory is missing some source files, or 
some preprocessing stage before actual compilation is adversely affected by 
ccache - some source files are not placed where build rules are expecting 
them to be, so this does not look like simple cache corruption issue.

Build logs and some more information are available here : 
https://bugs.gentoo.org/show_bug.cgi?id=512484

Completely disabling ccache makes the build proceed correctly. Cleaning the 
ccache caches doesn't help.

I'd like to know how can i more closely narrow down the problem in 
question, since debugging ccache/compilers is not something i am familiar 
with.