Workaround for installation issue from 1.3.1 release

Submodules commits taken from:
jinja2cpp#250

.gitmodules

@@ -0,0 +1,30 @@
+[submodule "thirdparty/gtest"]
+    path = thirdparty/gtest
+    url = https://github.com/google/googletest.git
+[submodule "thirdparty/boost"]
+	path = thirdparty/boost
+	url = https://github.com/boostorg/boost.git
+[submodule "thirdparty/nonstd/expected-lite"]
+	path = thirdparty/nonstd/expected-lite
+	url = https://github.com/martinmoene/expected-lite.git
+[submodule "thirdparty/nonstd/variant-lite"]
+	path = thirdparty/nonstd/variant-lite
+	url = https://github.com/martinmoene/variant-lite.git
+[submodule "thirdparty/nonstd/optional-lite"]
+	path = thirdparty/nonstd/optional-lite
+	url = https://github.com/martinmoene/optional-lite.git
+[submodule "thirdparty/nonstd/string-view-lite"]
+	path = thirdparty/nonstd/string-view-lite
+	url = https://github.com/martinmoene/string-view-lite.git
+[submodule "thirdparty/fmtlib"]
+	path = thirdparty/fmtlib
+	url = https://github.com/fmtlib/fmt.git
+[submodule "thirdparty/json/nlohmann"]
+	path = thirdparty/json/nlohmann
+	url = https://github.com/nlohmann/json.git
+[submodule "thirdparty/json/rapid"]
+	path = thirdparty/json/rapid
+	url = https://github.com/Tencent/rapidjson.git
+[submodule "fmt"]
+	path = fmt
+	url = https://github.com/fmtlib/fmt.git