Reinstate a Cygwin workround that was backed out by the previous changes.

Not all platforms support (or require) the -r argument to sed.

config.mk

@@ -0,0 +1,2 @@
+BOARD=mega
+PORT=/dev/null

libraries/AP_Common/Arduino.mk

@@ -259,7 +259,12 @@ SKETCHCPP_SRC		:=	$(SKETCHPDE) $(sort $(filter-out $(SKETCHPDE),$(SKETCHPDESRCS)
 # make.
 #
 SEXPR			=	's/^[[:space:]]*\#include[[:space:]][<\"]([^>\"./]+).*$$/\1/p'
-LIBTOKENS		:=	$(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nre $(SEXPR)))
+ifneq ($(findstring CYGWIN, $(SYSTYPE)),)
+  # Workaround a cygwin issue
+  LIBTOKENS        :=    $(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nre $(SEXPR)))
+else
+  LIBTOKENS        :=    $(sort $(shell cat $(SKETCHPDESRCS) $(SKETCHSRCS) | sed -nEe $(SEXPR)))
+endif
 
 #
 # Find sketchbook libraries referenced by the sketch.