Split -isystem headers into separate variables

Split the variables that contain header directories into ones that
should be prefixed with -isystem and ones that should be prefixed with
-I in preparation for moving some headers from -isystem to -I.

Add $(wildcard) around SRC_HEADERS to match the soong behavior, and move
users of SRC_HEADERS from config.mk to binary.mk so that the exported
soong value is present.

Test: no changes to build.ninja compile rules
Change-Id: Iadecbbf4351a01e53cb57e721d31f4f836bb82d9
diff --git a/core/definitions.mk b/core/definitions.mk
index 6bd9cf0..afb4558 100644
--- a/core/definitions.mk
+++ b/core/definitions.mk
@@ -1156,9 +1156,12 @@
 $(addprefix -I , $(PRIVATE_C_INCLUDES)) \
 $$(cat $(PRIVATE_IMPORT_INCLUDES))\
 $(if $(PRIVATE_NO_DEFAULT_COMPILER_FLAGS),,\
+    $(addprefix -I ,\
+        $(filter-out $(PRIVATE_C_INCLUDES), \
+            $(PRIVATE_GLOBAL_C_INCLUDES))) \
     $(addprefix -isystem ,\
         $(filter-out $(PRIVATE_C_INCLUDES), \
-            $(PRIVATE_GLOBAL_C_INCLUDES))))
+            $(PRIVATE_GLOBAL_C_SYSTEM_INCLUDES))))
 endef
 
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