Set includes as system interface to suppress warnings for other users (#415)

Set includes as system interface to suppress warnings for other users

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

cmake/ftxui_set_options.cmake

@@ -13,14 +13,31 @@ function(ftxui_set_options library)
     set_target_properties(${library}
       PROPERTIES CXX_CLANG_TIDY "${CLANG_TIDY_EXE};-warnings-as-errors=*"
     )
-  endif()
 
+    # By using "PUBLIC" as opposed to "SYSTEM INTERFACE", the compiler warning
+    # are enforced on the headers. This is behind "FTXUI_CLANG_TIDY", so that it
+    # applies only when developing FTXUI and on the CI. User's of the library
+    # get only the SYSTEM INTERFACE instead.
     target_include_directories(${library}
       PUBLIC
-      $<INSTALL_INTERFACE:include>
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+    )
+  else()
+    target_include_directories(${library} SYSTEM
+      INTERFACE
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+    )
+  endif()
+
+  target_include_directories(${library} SYSTEM
+    INTERFACE
+      $<INSTALL_INTERFACE:include>
+  )
+
+  target_include_directories(${library}
     PRIVATE
-      src
+      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/src>
+      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
   )
 
   # C++17 is used. We require fold expression at least.