Add version constraint for building against external library

Author: dopuskh3

external_zstd.go

@@ -5,4 +5,10 @@ package zstd
 
 // #cgo CFLAGS: -DUSE_EXTERNAL_ZSTD
 // #cgo pkg-config: libzstd
+/*
+#include<zstd.h>
+#if ZSTD_VERSION_NUMBER < 100400000
+#error "ZSTD version >= 1.4 is required ZSTD_VERSION_NUMBER"
+#endif
+*/
 import "C"