Converting to better_enums to support gcc 8

cmake/better-enums.cmake

@@ -0,0 +1,15 @@
+include(ExternalProject)
+
+project(better-enums)
+
+ExternalProject_Add(${PROJECT_NAME}-external
+        GIT_REPOSITORY https://github.com/aantron/better-enums.git
+        GIT_TAG "0.11.3"
+        GIT_SHALLOW TRUE
+        SOURCE_DIR ${THIRD_PARTY_DIR}/${PROJECT_NAME}
+        BUILD_COMMAND ""
+        INSTALL_COMMAND ""
+        CONFIGURE_COMMAND "")
+add_library(${PROJECT_NAME} INTERFACE)
+add_dependencies(${PROJECT_NAME} ${PROJECT_NAME}-external)
+target_include_directories(${PROJECT_NAME} SYSTEM INTERFACE ${THIRD_PARTY_DIR}/${PROJECT_NAME})