mk: remove include paths from make.inc

Now that the includes directives are stardardized, we don't need to add include
paths from make.inc anymore.

mk/sketch_sources.mk

@@ -31,7 +31,7 @@ SKETCHLIBNAMES		:=	$(notdir $(SKETCHLIBS))
 SKETCHLIBSRCDIRS	:=	$(SKETCHLIBS) $(addsuffix /utility,$(SKETCHLIBS))
 SKETCHLIBSRCS		:=	$(wildcard $(foreach suffix,$(SRCSUFFIXES),$(addsuffix /$(suffix),$(SKETCHLIBSRCDIRS))))
 SKETCHLIBOBJS		:=	$(addsuffix .o,$(basename $(subst $(SKETCHBOOK),$(BUILDROOT),$(SKETCHLIBSRCS))))
-SKETCHLIBINCLUDES	+=	$(addprefix -I,$(SKETCHLIBS))
+SKETCHLIBINCLUDES	:=	-I$(SKETCHBOOK)/libraries/
 SKETCHLIBSRCSRELATIVE	:=	$(subst $(SKETCHBOOK)/,,$(SKETCHLIBSRCS))
 
 ifeq ($(VERBOSE),)