Patch json library to support older versions of gcc (#598)

* patch working

* wip

* fix patcher

* remove debug message:

* cleanup

* fix typo

cmake/Modules/patch_json.cmake

+set(FILE_NAME ${CMAKE_BINARY_DIR}/include/nlohmann/detail/macro_scope.hpp)
+file(READ ${FILE_NAME} FILE_CONTENTS)
+string(REPLACE
+  "#if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40900"
+  "#if (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40805"
+  REWRITTEN_FILE
+  "${FILE_CONTENTS}"
+)
+file(WRITE ${FILE_NAME} "${REWRITTEN_FILE}")
+message(STATUS "json library gcc minimum version number patched")

cmake/external_json.cmake

@@ -34,6 +34,9 @@ if (${CMAKE_VERSION} VERSION_LESS 3.2)
         BUILD_COMMAND ""
         INSTALL_COMMAND ""
         UPDATE_COMMAND ""
+        # cmake does not allow calling cmake functions so we call a cmake script in the Module
+        # directory.
+        PATCH_COMMAND ${CMAKE_COMMAND} -P ${CMAKE_MODULE_PATH}patch_json.cmake
         )
 else()
     ExternalProject_Add(
@@ -44,7 +47,10 @@ else()
         BUILD_COMMAND ""
         INSTALL_COMMAND ""
         UPDATE_COMMAND ""
-        )
+        # cmake does not allow calling cmake functions so we call a cmake script in the Module
+        # directory.
+        PATCH_COMMAND ${CMAKE_COMMAND} -P ${CMAKE_MODULE_PATH}patch_json.cmake
+    )
 endif()
 
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