libmakepkg: set CCACHE_PREFIX to absolute path of distcc when using both

The current logic sets CCACHE_PREFIX to distcc when both distcc and ccache are enabled. However, according to the source of ccache, it would execute the command with execv, which would not look up arg0 from PATH, unlike those exec functions with _p suffix.

This would result in the following error, when building a package with both ccache and distcc enabled:

ccache: error: execute_noreturn of distcc failed: No such file or directory

This breaks package builds in different ways: packages that use make/cc directly would yield the actual error which is the same as the above line, packages that rely on other build systems wouldn't go through compiler check and complain on a bad compiler.

To reproduce the problem, try to build a simple package:

git clone https://gitlab.archlinux.org/archlinux/packaging/packages/abc.git
cd abc
cp /etc/makepkg.conf .
echo 'BUILDENV=(distcc color ccache check !sign)' >> makepkg.conf
makepkg --config makepkg.conf

refs: https://github.com/ccache/ccache/blob/f887434d35bf6544ce3b56dd7684de804cdaacfa/src/ccache/execute.cpp#L348 https://man.archlinux.org/man/exec.3.en#v_-_execv(),_execvp(),_execvpe()

Signed-off-by: Guoxin Pu pugokushin@gmail.com