[Libreoffice] ccache support

[Norbert Thiebaud]

I've pushed
http://cgit.freedesktop.org/libreoffice/core/commit/?id=57cf026739a3d707378ca38f59518b018fccca8f

This try to automatically use ccache when present, unless CC= or CXX=
was specified by the user or --disable-ccache was passed to autogen
This also detect if CC or CXX is already an alias for ccache, in which
case ccache is not prefixed to CC and/or CXX

Windows VC++ should not be concerned by these changes.... (unless I
messed-up :-) )

Norbert