Add --with-libacl for full permission checking

libacl is only mandatory for Cygwin and must be explicitly selected for
other platforms. For now, expressly binding to libacl in order to have
acl_get_perm, rather than adding general ACL support (see libarchive for
the full horrendous details of how to do this portably otherwise).

.gitignore

@@ -74,6 +74,8 @@ src/client/linking.sexp
 src/core/developer
 src/manifest/dune
 src/manifest/install.inc
+src/stubs/libacl/dune
+src/stubs/libacl/c-libraries.sexp
 src/stubs/win32/cc64
 src/stubs/win32/dune
 src/stubs/c-flags.sexp

Makefile

@@ -79,7 +79,7 @@ clean:
 distclean: clean clean-ext
 	rm -rf autom4te.cache bootstrap
 	rm -f Makefile.config config.log config.status aclocal.m4
-	rm -f src/*.META src/*/.merlin src/manifest/dune src/manifest/install.inc src/stubs/win32/dune src/stubs/win32/cc64 src/ocaml-flags-configure.sexp
+	rm -f src/*.META src/*/.merlin src/manifest/dune src/manifest/install.inc src/stubs/libacl/dune src/stubs/win32/dune src/stubs/win32/cc64 src/ocaml-flags-configure.sexp src/stubs/libacl/c-libraries.sexp
 	rm -f src/client/linking.sexp src/stubs/c-flags.sexp src/core/developer src/core/version
 
 OPAMINSTALLER_FLAGS = --prefix "$(DESTDIR)$(prefix)"