Instead of running compilers directly, let them go through the native
ccache binary. Note that it will still use the foreign arches cache dir
because the native cache dir would be at /native/home/pmos/.ccache.
Use execve instead of setenv and execv, because that didn't work with
LD_LIBRARY_PATH (ccache was complaining that it couldn't load zlib).
Enable -Werror, because we are maintaining this program and it helps
us catching bugs early.
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