Build broken with old zstd

[Christophe Lyon]

Hi!

I have noticed that the debian-armhf buildbot on Sourceware 
(https://builder.sourceware.org/buildbot/#/builders?tags=binutils)
has been failing to build binutils for quite a long time, with:
../../binutils-gdb/gas/compress-debug.c: In function ‘compress_data’:
../../binutils-gdb/gas/compress-debug.c:62:20: error: implicit 
declaration of function ‘ZSTD_compressStream2’; did you mean 
‘ZSTD_compressStream’? [-Werror=implicit-function-declaration]
        size_t ret = ZSTD_compressStream2 (ctx, &ob, &ib, ZSTD_e_continue);
                     ^~~~~~~~~~~~~~~~~~~~
                     ZSTD_compressStream


I took me a bit of time to reproduce it, because the build succeeded 
with debian stable, I had to switch to oldstable to see the failure 
(IIUC that's the version used by the buildbot)

My understanding is that the version of zstd shipped with debian 
oldstable is too old for binutils (it does not provide 
ZSTD_compressStream2).
I did not see the problem in my ubuntu containers because I do not 
install libzstd-dev there, and configure disables the use of zstd.

I am surprised no one has noticed this before, but I see several other 
Sourceware binutils builbots are failing on other targets, maybe for the 
same reason.

It looks like the right fix is to improve the detection of zstd at 
configure time (IIUC we only check if it's present, not its version).

Should I file a PR for someone to look at this?

Thanks,

Christophe