checkheaders: Fix directory creation in out-of-tree builds.

checkheaders wasn't creating folders as needed, so if it was run
immediately after the configure script it would fail as soon as
it tried to compile headers inside the ARCH folders.
Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

common.mak

@@ -97,7 +97,8 @@ DEP_LIBS := $(foreach NAME,$(FFLIBS),lib$(NAME)/$($(CONFIG_SHARED:yes=S)LIBNAME)
 ALLHEADERS := $(subst $(SRC_DIR)/,$(SUBDIR),$(wildcard $(SRC_DIR)/*.h $(SRC_DIR)/$(ARCH)/*.h))
 SKIPHEADERS += $(ARCH_HEADERS:%=$(ARCH)/%) $(SKIPHEADERS-)
 SKIPHEADERS := $(SKIPHEADERS:%=$(SUBDIR)%)
-checkheaders: $(filter-out $(SKIPHEADERS:.h=.ho),$(ALLHEADERS:.h=.ho))
+HEADEROBJS  := $(filter-out $(SKIPHEADERS:.h=.ho),$(ALLHEADERS:.h=.ho))
+checkheaders:  $(HEADEROBJS)
 
 alltools: $(TOOLS)
 
@@ -110,9 +111,10 @@ $(HOSTPROGS): %$(HOSTEXESUF): %.o
 $(OBJS):       | $(sort $(dir $(OBJS)))
 $(HOSTOBJS):   | $(sort $(dir $(HOSTOBJS)))
 $(TESTOBJS):   | $(sort $(dir $(TESTOBJS)))
+$(HEADEROBJS): | $(sort $(dir $(HEADEROBJS)))
 $(TOOLOBJS):   | tools
 
-OBJDIRS := $(OBJDIRS) $(dir $(OBJS) $(HOSTOBJS) $(TESTOBJS))
+OBJDIRS := $(OBJDIRS) $(dir $(OBJS) $(HOSTOBJS) $(TESTOBJS) $(HEADEROBJS))
 
 CLEANSUFFIXES     = *.d *.o *~ *.ho *.map *.ver *.gcno *.gcda
 DISTCLEANSUFFIXES = *.pc