[Makefile] try to install to /etc/bash_completion.d only when the actual user can write to it

Makefile.in

@@ -1328,7 +1328,7 @@ clean::
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 install_no_local::
-	if test -d /etc/bash_completion.d; then cp -f share/bash/why3 /etc/bash_completion.d; fi
+	if test -d /etc/bash_completion.d -a -w /etc/bash_completion.d; then cp -f share/bash/why3 /etc/bash_completion.d; fi
 
 
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