Update "JSON for Modern C++" to a version that officially supports gc… (#1955)

* Update "JSON for Modern C++" to a version that officially supports gcc 4.8

* "JSON for Modern C++" is header only and does not need to be configured.

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

@@ -22,40 +22,20 @@ include(ExternalProject)
 #------------------------------------------------------------------------------
 
 SET(JSON_GIT_REPO_URL https://github.com/nlohmann/json)
-SET(JSON_GIT_LABEL v3.1.1)
+SET(JSON_GIT_LABEL v3.4.0)
 
-# The 'BUILD_BYPRODUCTS' argument was introduced in CMake 3.2.
-if (${CMAKE_VERSION} VERSION_LESS 3.2)
-    ExternalProject_Add(
+ExternalProject_Add(
     ext_json
     PREFIX json
     GIT_REPOSITORY ${JSON_GIT_REPO_URL}
     GIT_TAG ${JSON_GIT_LABEL}
     # Disable install step
+    CONFIGURE_COMMAND ""
     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_SOURCE_DIR}/cmake/Modules/patch_json.cmake
     EXCLUDE_FROM_ALL TRUE
     )
-else()
-    ExternalProject_Add(
-        ext_json
-        PREFIX json
-        GIT_REPOSITORY ${JSON_GIT_REPO_URL}
-        GIT_TAG ${JSON_GIT_LABEL}
-        # Disable install step
-        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_SOURCE_DIR}/cmake/Modules/patch_json.cmake
-        EXCLUDE_FROM_ALL TRUE
-    )
-endif()
 
 #------------------------------------------------------------------------------