variant support from Commit 07f4293 of original branch

Author: kolbbond

Arduino/System/BoardBuildTargets.cmake

@@ -768,14 +768,37 @@ function(_add_internal_arduino_core target)
 
 	# get_headers_parent_directories("${core_headers};${variant_headers}" include_dirs)
 
-	# Add the library and set the include directories
-	add_library("${target}" STATIC ${core_headers} ${core_sources}
-		${variant_headers} ${variant_sources})
-	# target_include_directories(${target} PUBLIC ${include_dirs})
-	target_include_directories(${target} PUBLIC
-		"${ARDUINO_BOARD_BUILD_CORE_PATH}"
-		"${ARDUINO_BOARD_BUILD_VARIANT_PATH}")
+    # variant source
+    if (NOT ${variant_sources} STREQUAL "" AND TRUE)
+            add_library("${target}_vsources_" OBJECT
+                ${variant_headers}
+                ${variant_sources})
+            target_include_directories("${target}_vsources_" PUBLIC
+                "${ARDUINO_BOARD_BUILD_VARIANT_PATH}"
+                "${ARDUINO_BOARD_BUILD_CORE_PATH}")
+            # Add the library and set the include directories
+            add_library("${target}" STATIC ${variant_headers}
+                ${core_headers} ${core_sources})
+            # target_include_directories(${target} PUBLIC ${include_dirs})
+            target_include_directories(${target} PUBLIC
+                "${ARDUINO_BOARD_BUILD_VARIANT_PATH}"
+                "${ARDUINO_BOARD_BUILD_CORE_PATH}")
+            # Add the object library to the sources of the linking targets
+            # Should work above CMake 3.1, but GENERATED property does not seem
+            # to be working well causing the portable_app example (transitive
+            # linking o core) to not to link with this in the first run.
+            set_target_properties(${target} PROPERTIES
+                INTERFACE_SOURCES  "$<TARGET_OBJECTS:${target}_vsources_>")
+        else()
+            # Add the library and set the include directories
+            add_library("${target}" STATIC ${variant_headers}
+                ${core_headers} ${core_sources})
+            # target_include_directories(${target} PUBLIC ${include_dirs})
+            target_include_directories(${target} PUBLIC
+                "${ARDUINO_BOARD_BUILD_VARIANT_PATH}"
+                "${ARDUINO_BOARD_BUILD_CORE_PATH}")
 
+	endif()
 	# Add the prebuild and postbuild command hooks for the core
 	_set_arduino_target_hooks("${target}" "core.prebuild" PRE_BUILD)
 	_set_arduino_target_hooks("${target}" "core.postbuild" POST_BUILD)