Improved callback/states support to make libxbps std{err,out} printf free.

This is a major API/ABI change, documentation has been updated accordingly.

include/compat.h

@@ -2,6 +2,7 @@
 #define COMPAT_H
 
 #include <sys/types.h>
+#include <stdarg.h>
 
 #ifndef HAVE_STRLCAT
 size_t strlcat(char *, const char *, size_t);
@@ -15,4 +16,8 @@ size_t strlcpy(char *, const char *, size_t);
 char *strcasestr(const char *, const char *);
 #endif
 
+#if defined(HAVE_VASPRINTF) && !defined(_GNU_SOURCE)
+int vasprintf(char **, const char *, va_list);
+#endif
+
 #endif /* COMPAT_H */