set RPATH (only to be used on ELF platforms)

diff -ru python3.orig/Makefile.pre.in python3/Makefile.pre.in
--- python3.orig/Makefile.pre.in	2015-07-26 20:29:07.126194320 +0200
+++ python3/Makefile.pre.in	2015-07-26 20:37:21.814227530 +0200
@@ -550,7 +550,7 @@
 
 # Build the interpreter
 $(BUILDPYTHON):	Programs/python.o $(LIBRARY) $(LDLIBRARY) $(PY3LIBRARY)
-	$(LINKCC) $(PY_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/python.o $(BLDLIBRARY) $(LIBS) $(MODLIBS) $(SYSLIBS) $(LDLAST)
+	$(LINKCC) $(PY_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/python.o $(BLDLIBRARY) $(LIBS) $(MODLIBS) $(SYSLIBS) $(LDLAST) -Wl,-rpath,\$$ORIGIN
 
 platform: $(BUILDPYTHON) pybuilddir.txt
 	$(RUNSHARED) $(PYTHON_FOR_BUILD) -c 'import sys ; from sysconfig import get_platform ; print(get_platform()+"-"+sys.version[0:3])' >platform