Allow user to pass code generation input to Makefile

Makefile

@@ -7,6 +7,7 @@ AR             = ar
 BUILD_TYPE     = Debug
 BUILD_DIR      = libwapp-build/$(PLATFORM)-$(BUILD_TYPE)
 INSTALL_PREFIX = dist
+CODEGEN_INPUT  = ""
 
 # Internal variables
 override CFLAGS              = -Wall -Wextra -Werror -pedantic -Isrc
@@ -54,6 +55,7 @@ help:
 	@echo "  BUILD_TYPE        Build type. Choose from \`Debug\`, \`RelWithDebInfo\` or \`Release\` (Default: Debug)."
 	@echo "  BUILD_DIR         Directory where build files will be written."
 	@echo "  INSTALL_PREFIX    Prefix where library and include files will be installed."
+	@echo "  CODEGEN_INPUT     Input file for code generation (See codegen_custom_data_example.json for an example)."
 	@echo
 	@echo "Available targets:"
 	@echo "  make              Build, install and test the full wapp library."
@@ -113,7 +115,7 @@ run-cc-test: build-cc-test
 	@rm $(TEST_CXX_OUT)
 
 codegen:
-	python3 -m codegen
+	python3 -m codegen -f $(CODEGEN_INPUT)
 
 install: build-lib
 	@mkdir -p $(LIB_INSTALL)