GNU sed wants -r, Darwin/BSD sed wants -E.

libraries/AP_Common/Arduino.mk

@@ -259,11 +259,10 @@ SKETCHCPP_SRC		:=	$(SKETCHPDE) $(sort $(filter-out $(SKETCHPDE),$(SKETCHPDESRCS)
 # make.
 #
 SEXPR			=	's/^[[:space:]]*\#include[[:space:]][<\"]([^>\"./]+).*$$/\1/p'
-ifneq ($(findstring CYGWIN, $(SYSTYPE)),)
-  # Workaround a cygwin issue
-  LIBTOKENS        :=    $(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nre $(SEXPR)))
-else
+ifeq ($(SYSTYPE),Darwin)
   LIBTOKENS        :=    $(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nEe $(SEXPR)))
+else
+  LIBTOKENS        :=    $(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nre $(SEXPR)))
 endif
 
 #