Expose include directory as system path via CMake

Treading the include directory as a system include directory prevents
clang-tidy from emitting errors from the doctest headers, avoiding
issues when using various doctest macros.

CMakeLists.txt

@@ -50,7 +50,7 @@ if(MAIN_PROJECT)
 
     add_custom_target(assemble_single_header ALL DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/doctest/doctest.h)
 else()
-    target_include_directories(${PROJECT_NAME} INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/>)
+    target_include_directories(${PROJECT_NAME} SYSTEM INTERFACE $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/>)
 endif()
 
 if(CMAKE_BUILD_TYPE STREQUAL "Debug")