CMake: When building metalib components, properly flag SYSTEM include dirs

cmake/macros/BuildMetalib.cmake

@@ -31,6 +31,12 @@ if(CMAKE_VERSION VERSION_LESS "3.12")
         set_property(TARGET "${target}" APPEND PROPERTY INCLUDE_DIRECTORIES "${include_directories}")
         set_property(TARGET "${target}" APPEND PROPERTY INTERFACE_INCLUDE_DIRECTORIES "${include_directories}")
 
+        # SYSTEM include directories should still be reported as SYSTEM, so
+        # that warnings from those includes are suppressed
+        set(sys_include_directories
+          "$<TARGET_PROPERTY:${library},INTERFACE_SYSTEM_INCLUDE_DIRECTORIES>")
+        target_include_directories("${target}" SYSTEM PUBLIC "${sys_include_directories}")
+
         # And for INTERFACE_COMPILE_DEFINITIONS as well
         set(compile_definitions "$<TARGET_PROPERTY:${library},INTERFACE_COMPILE_DEFINITIONS>")
         set_property(TARGET "${target}" APPEND PROPERTY COMPILE_DEFINITIONS "${compile_definitions}")

cmake/macros/PackageConfig.cmake

@@ -158,6 +158,18 @@ function(package_option name)
   # Create the INTERFACE library used to depend on this package.
   add_library(PKG::${name} INTERFACE IMPORTED GLOBAL)
 
+  # Explicitly record the package's include directories as system include
+  # directories.  CMake does do this automatically for INTERFACE libraries, but
+  # it does it by discovering all transitive links first, then reading
+  # INTERFACE_INCLUDE_DIRECTORIES for those which are INTERFACE libraries.  So,
+  # this would be broken for the metalib system (pre CMake 3.12) which doesn't
+  # "link" the object libraries.
+  if(CMAKE_VERSION VERSION_LESS "3.12")
+    set_target_properties(PKG::${name} PROPERTIES
+      INTERFACE_SYSTEM_INCLUDE_DIRECTORIES
+      "$<TARGET_PROPERTY:PKG::${name},INTERFACE_INCLUDE_DIRECTORIES>")
+  endif()
+
   # Create the option, and if it actually is enabled, populate the INTERFACE
   # library created above
   option("HAVE_${name}" "${cache_string}" "${default}")