Implemented GH #39

The system repository directory set to <prefix>/share/xbps/repo.d contains
system repository configuration files (.conf) that can be overrided by the admin
in <sysconfdir>/xbps/repo.d bearing the same file name.

include/Makefile

@@ -4,7 +4,7 @@ INCS =	xbps.h
 
 .PHONY: all
 all:
-	sed -e "s|@@VERSION@@|${VERSION}|g" ${INCS}.in > ${INCS}
+	sed -e "s|@@VERSION@@|${VERSION}|g;s|@@PREFIX@@|${PREFIX}|g" ${INCS}.in > ${INCS}
 
 .PHONY: install
 install: