Adds Pybind as a submodule and uses v2.6

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "python_bindings/pybind11"]
+	path = python_bindings/pybind11
+	url = https://github.com/pybind/pybind11

CMakeLists.txt

@@ -108,9 +108,31 @@ add_subdirectory(test)
 add_subdirectory(tools)
 add_subdirectory(apps)
 string(REPLACE " -Wmissing-declarations" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+
+find_package(Git QUIET)
+if(GIT_FOUND AND EXISTS "${TACO_PROJECT_DIR}/.git")
+  # Update submodules as needed
+  option(GIT_SUBMODULE "Check submodules during build" ON)
+  if(GIT_SUBMODULE)
+    message(STATUS "Submodule update")
+    execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
+            WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+            RESULT_VARIABLE GIT_SUBMOD_RESULT)
+    if(NOT GIT_SUBMOD_RESULT EQUAL "0")
+      message(FATAL_ERROR "git submodule update --init failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+    endif()
+  endif()
+endif()
+
+if(NOT EXISTS "${TACO_PROJECT_DIR}/python_bindings/pybind11/CMakeLists.txt")
+  message(FATAL_ERROR "The submodules were not downloaded! GIT_SUBMODULE was turned off or failed. Please update submodules and try again.")
+endif()
+
 if(PYTHON)
-  add_subdirectory(python_bindings)
+  message("-- Will build Python extension")
+  add_definitions(-DPYTHON)
 endif(PYTHON)
+
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wmissing-declarations")
 add_custom_target(src DEPENDS apps)