Make Issue Unidata#2077 work when build is repeated.

re: Unidata#2075

The long file name fix fails when the build is manually repeated
because the source file has already been renamed. Solution is to
test if the dest file exists or not before doing the rename.
This is apparently not a problem for automake because it uses
AC_CONFIG_LINK, which uses link/copy instead of rename.

RELEASE_NOTES.md

@@ -7,6 +7,8 @@ This file contains a high-level description of this package's evolution. Release
 
 ## 4.8.2 - TBD
 
+* [Bug Fix] Make PR 2075 long file name be idempotent. See [Github #2066](https://github.com/Unidata/netcdf-c/pull/2066).
+
 ## 4.8.1 - August 18, 2021
 
 * [Bug Fix] Fix multiple bugs in libnczarr. See [Github #2066](https://github.com/Unidata/netcdf-c/pull/2066).