added subrepos

Former-commit-id: f738078b74194aadcff609807b043909e8ef7d55

.gitmodules

@@ -12,3 +12,12 @@ url=https://github.com/libigl/nanogui.git
 [submodule "external/cgal"]
 	path = external/cgal
 	url = https://github.com/CGAL/cgal.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