Allow for ergnomically putting Unix-only files in subdirs by creating INLCUDE_$(pkg) vars

Separate platform-specific files will allow avoiding a lot of CPP.

tests/unit/libexpr/local.mk

@@ -23,10 +23,10 @@ libexpr-tests_EXTRA_INCLUDES = \
     -I tests/unit/libexpr-support \
     -I tests/unit/libstore-support \
     -I tests/unit/libutil-support \
-    -I src/libexpr \
-    -I src/libfetchers \
-    -I src/libstore \
-    -I src/libutil
+    $(INCLUDE_libexpr) \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libutil)
 
 libexpr-tests_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)