python: Ignore installed when installing to DESTDIR

When installing to a destdir with pip install --prefix= --root=, pip tries to
uninstall the existing root-owned package and fails

Fixes:
python3 -m pip install --prefix=/usr `test -n "/tmp/selinux-release//build-master" && echo --root /tmp/selinux-release//build-master`  .
Processing /tmp/selinux-release/selinux-master/python/sepolicy
  Preparing metadata (setup.py) ... done
Building wheels for collected packages: sepolicy
  Building wheel for sepolicy (setup.py) ... done
  Created wheel for sepolicy: filename=sepolicy-3.4-py3-none-any.whl size=1663564 sha256=229546db123e7d84613d190d49c192291b1a4f7f2a037657b39283b04ac391a4
  Stored in directory: /tmp/pip-ephem-wheel-cache-50r2x4cn/wheels/b2/9e/63/6a6212a84d65a709923228719d065ed34e66a90c7fed01e8cf
Successfully built sepolicy
Installing collected packages: sepolicy
  Attempting uninstall: sepolicy
    Found existing installation: sepolicy 3.4
    Uninstalling sepolicy-3.4:
ERROR: Could not install packages due to an OSError: [Errno 13] Permission denied: 'generate.py'
Consider using the `--user` option or check the permissions.

Signed-off-by: Jason Zaman <[email protected]>
Acked-by: Petr Lautrbach <[email protected]>

python/sepolicy/Makefile

@@ -27,7 +27,7 @@ test:
 	@$(PYTHON) test_sepolicy.py -v
 
 install:
-	$(PYTHON) -m pip install --prefix=$(PREFIX) `test -n "$(DESTDIR)" && echo --root $(DESTDIR)` $(PYTHON_SETUP_ARGS) .
+	$(PYTHON) -m pip install --prefix=$(PREFIX) `test -n "$(DESTDIR)" && echo --root $(DESTDIR) --ignore-installed --no-deps` $(PYTHON_SETUP_ARGS) .
 	[ -d $(DESTDIR)$(BINDIR) ] || mkdir -p $(DESTDIR)$(BINDIR)
 	install -m 755 sepolicy.py $(DESTDIR)$(BINDIR)/sepolicy
 	(cd $(DESTDIR)$(BINDIR); ln -sf sepolicy sepolgen)