[ccache] [PATCH] Support for Clang precompiled headers

[Lubos Lunak]

 Hello,

 the attached patches modify ccache to also support Clang PCH in addition to 
GCC PCH.

 The changes are:

- Clang uses .pch as the extension, not .gch
- the .pch file can be directly passed using -include-pch, instead of 
using -include and trying to find the .pch file
- the use of a .pch is not reflected in the resulting .ii , so it needs to be 
explicitly added to the hash for checking if used files have changed
- I also believe that path_len passed to remember_include_file() is incorrect, 
as make_relative_path() may change the string

PS: Since I've noticed in the archives the recent mail about issues with 
ccache and warnings about unused arguments: The proper way to use ccache with 
Clang is to set CCACHE_CPP2, which not only avoids these warnings, but in 
general Clang works suboptimally if passed preprocessed output (warning/error 
messages quoting sources are affected, some warnings are not supressed in 
headers).

-- 
 Lubos Lunak
 l.lunak at suse.cz
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