Fix python Requires in Fedora 17 build

When python's distutils build process prepares python scripts, it use
current Python interpreter in an updated shebang for python scripts.
Since Makefile did not use absolute path to python interpreter, it
may be translated to "/bin/python" in Fedora 17 which is then taken
by rpmbuild as freeipa-admintools dependency. This can break of F-17
python package which provides just "/usr/bin/python"

This patch updates Makefile to use a correct absolute path to python
interpreter which is then filled to freeipa scripts shebang and rpm
Requires list. The value is taken from RPM __python macro so that
we do not hardcode it.

https://fedorahosted.org/freeipa/ticket/2727

Makefile

@@ -44,6 +44,8 @@ ifneq ($(DEVELOPER_MODE),0)
 LINT_OPTIONS=--no-fail
 endif
 
+PYTHON ?= $(shell rpm -E %__python)
+
 all: bootstrap-autogen server
 	@for subdir in $(SUBDIRS); do \
 		(cd $$subdir && $(MAKE) $@) || exit 1; \
@@ -74,9 +76,9 @@ client-install: client client-dirs
 	done
 	cd install/po && $(MAKE) install || exit 1;
 	if [ "$(DESTDIR)" = "" ]; then \
-		python setup-client.py install; \
+		$(PYTHON) setup-client.py install; \
 	else \
-		python setup-client.py install --root $(DESTDIR); \
+		$(PYTHON) setup-client.py install --root $(DESTDIR); \
 	fi
 
 client-dirs:
@@ -132,13 +134,13 @@ version-update: release-update
 	fi
 
 server: version-update
-	python setup.py build
+	$(PYTHON) setup.py build
 
 server-install: server
 	if [ "$(DESTDIR)" = "" ]; then \
-		python setup.py install; \
+		$(PYTHON) setup.py install; \
 	else \
-		python setup.py install --root $(DESTDIR); \
+		$(PYTHON) setup.py install --root $(DESTDIR); \
 	fi
 
 archive: