Use JSON_TestDataDirectory directly

Author: LocutusOfBorg

cmake/download_test_data.cmake

@@ -1,19 +1,14 @@
 set(JSON_TEST_DATA_URL     https://github.com/nlohmann/json_test_data)
 set(JSON_TEST_DATA_VERSION 3.1.0)
 
-set(JSON_TEST_DATA_DIR /usr/share/json_test_data)
-
-if(EXISTS ${JSON_TEST_DATA_DIR})
-  set(JSON_TestDataDirectory ${JSON_TEST_DATA_DIR})
-endif()
+include(ExternalProject)
 
 # if variable is set, use test data from given directory rather than downloading them
 if(JSON_TestDataDirectory)
     message(STATUS "Using test data in ${JSON_TestDataDirectory}.")
     add_custom_target(download_test_data)
     file(WRITE ${CMAKE_BINARY_DIR}/include/test_data.hpp "#define TEST_DATA_DIRECTORY \"${JSON_TestDataDirectory}\"\n")
 else()
-    include(ExternalProject)
     # create a header with the path to the downloaded test data
     file(WRITE ${CMAKE_BINARY_DIR}/include/test_data.hpp "#define TEST_DATA_DIRECTORY \"${CMAKE_BINARY_DIR}/test_files\"\n")