Fix 263: wrong default path for COCCINELLE_HOME

COCCINELLE_HOME is the directory where standard.iso is looked for. If COCCINELLE_HOME is not defined, we consider the directory $bindir where the current executable is. If $bindir/standard.iso exists, we use COCCINELLE_HOME=$bindir (this is a usual case during development, where we run spatch.opt from the working directory of the repository).

Otherwise, we suppose that coccinelle has been installed (make install), and that standard.iso is installed in $libdir, where $libdir is $exec_prefix/lib.

Before this commit, we considered wrongly that $exec_prefix was equal to $bindir, whereas the default value for $bindir is $exec_prefix/bin. Therefore, we should take for $exec_prefix the parent directory of $bindir.