Update tests that generate CMake projects to use the CMAKE_CXX_COMPILER the main project was CMake'd with. Fixes #1747.

test/cmake_add_subdirectory/CMakeLists.txt

@@ -1,6 +1,7 @@
 add_test(NAME cmake_add_subdirectory_configure
   COMMAND ${CMAKE_COMMAND}
     -G "${CMAKE_GENERATOR}"
+    -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
     -Dnlohmann_json_source=${PROJECT_SOURCE_DIR}
     ${CMAKE_CURRENT_SOURCE_DIR}/project
 )

test/cmake_import/CMakeLists.txt

@@ -2,6 +2,7 @@ add_test(NAME cmake_import_configure
   COMMAND ${CMAKE_COMMAND}
     -G "${CMAKE_GENERATOR}"
     -A "${CMAKE_GENERATOR_PLATFORM}"
+    -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
     -Dnlohmann_json_DIR=${PROJECT_BINARY_DIR}
     ${CMAKE_CURRENT_SOURCE_DIR}/project
 )

test/cmake_import_minver/CMakeLists.txt

@@ -2,6 +2,7 @@ add_test(NAME cmake_import_minver_configure
   COMMAND ${CMAKE_COMMAND}
     -G "${CMAKE_GENERATOR}"
     -A "${CMAKE_GENERATOR_PLATFORM}"
+    -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
     -Dnlohmann_json_DIR=${PROJECT_BINARY_DIR}
     ${CMAKE_CURRENT_SOURCE_DIR}/project
 )