submodules fixed

Former-commit-id: 26bfb6db5bd230eae0d71610c8bb5a1c627b2380

.gitmodules

@@ -9,3 +9,15 @@ url=https://github.com/libigl/nanogui.git
 [submodule "external/pybind11"]
 	path = external/pybind11
 	url = https://github.com/wjakob/pybind11.git
+[submodule "external/tetgen"]
+	path = external/tetgen
+	url = https://github.com/libigl/tetgen.git
+[submodule "external/triangle"]
+	path = external/triangle
+	url = https://github.com/libigl/triangle.git
+[submodule "external/tinyxml2"]
+	path = external/tinyxml2
+	url = https://github.com/leethomason/tinyxml2.git
+[submodule "external/cgal"]
+	path = external/cgal
+	url = https://github.com/CGAL/cgal.git