build: Move setting of SRC_DIR to the only place it is used

common.mak

@@ -27,6 +27,7 @@ HEADERS   += $(HEADERS-yes)
 
 DEP_LIBS := $(foreach NAME,$(FFLIBS),lib$(NAME)/$($(CONFIG_SHARED:yes=S)LIBNAME))
 
+SRC_DIR    := $(SRC_PATH)/lib$(NAME)
 ALLHEADERS := $(subst $(SRC_DIR)/,$(SUBDIR),$(wildcard $(SRC_DIR)/*.h $(SRC_DIR)/$(ARCH)/*.h))
 SKIPHEADERS += $(ARCH_HEADERS:%=$(ARCH)/%) $(SKIPHEADERS-)
 SKIPHEADERS := $(SKIPHEADERS:%=$(SUBDIR)%)

library.mak

-SRC_DIR := $(SRC_PATH)/lib$(NAME)
-
 include $(SRC_PATH)/common.mak
 
 LIBVERSION := $(lib$(NAME)_VERSION)