Use a common place to set XBPS_VERSION.

include/Makefile

@@ -4,6 +4,7 @@ INCS =	xbps_api.h
 
 .PHONY: all
 all:
+	sed -e "s|@@VERSION@@|${VERSION}|g" ${INCS}.in > ${INCS}
 
 .PHONY: install
 install:
@@ -16,3 +17,4 @@ uninstall:
 
 .PHONY: clean
 clean:
+	-rm -f ${INCS}

include/xbps_api.h.in

@@ -57,7 +57,10 @@
 #define XBPS_PKGINDEX_VERSION	"1.5"
 
 #define XBPS_API_VERSION	"20120930"
-#define XBPS_VERSION		"0.17"
+
+#ifndef XBPS_VERSION
+ #define XBPS_VERSION		"0.17"
+#endif
 
 /**
  * @def XBPS_RELVER