Add a cmake option to use SYSTEM in target_include_directories

Without SYSTEM static analysis like clang-tidy raise warnings on
json.hpp.  This is not desirable for projects embedding nlohmann_json

Tests:
- Validated that without the option OFF the warnings are raised as
  before
- Validated that with the option ON the warnings are silenced

CMakeLists.txt

@@ -36,6 +36,7 @@ option(JSON_MultipleHeaders "Use non-amalgamated version of the library." OFF)
 option(JSON_ImplicitConversions "Enable implicit conversions." ON)
 option(JSON_Diagnostics "Enable better diagnostic messages." OFF)
 option(JSON_CI "Enable CI build targets." OFF)
+option(JSON_SystemInclude "Include as system headers (skip for clang-tidy)" OFF)
 
 if (JSON_CI)
     include(cmake/ci.cmake)
@@ -73,6 +74,10 @@ if (JSON_Diagnostics)
     message(STATUS "Diagnostics enabled")
 endif()
 
+if (JSON_SystemInclude)
+    set(NLOHMANN_JSON_SYSTEM_INCLUDE "SYSTEM")
+endif()
+
 ##
 ## TARGET
 ## create target and add include path
@@ -94,7 +99,7 @@ target_compile_definitions(
 
 target_include_directories(
     ${NLOHMANN_JSON_TARGET_NAME}
-    INTERFACE
+    ${NLOHMANN_JSON_SYSTEM_INCLUDE} INTERFACE
     $<BUILD_INTERFACE:${NLOHMANN_JSON_INCLUDE_BUILD_DIR}>
     $<INSTALL_INTERFACE:include>
 )