fix RUNPATH for wx libraries

For reasons unclear to me, the wx shared libraries have their RUNPATH set
to the library path of my _system_ python (not even the virtualenv python).
That's not going to work, especially when we package up and ship the
virtualenv.

Setting the RUNPATH to $ORIGIN says that the linker should look for
libwx_gtk3u_core-3.0.so.0 et al. in the same directory as _core.so.

Makefile

@@ -5,4 +5,5 @@ inkstitch-venv: requirements.txt
 	rm -rf inkstitch-venv
 	virtualenv inkstitch-venv
 	source inkstitch-venv/bin/activate \
-	pip install -r requirements.txt
+	pip install -r requirements.txt \
+    for file in inkstitch-venv/lib/python*/site-packages/wx/_*.so; do patchelf --set-rpath '$$ORIGIN'; done