Make ZSTD support work

[vejnar]

Hi,

Thanks for adding native Zstandard support. But current implementation doesn't work completely:

• the Make WITH_ZSTD flag isn't added to CXXFLAGS in the Makefile (so conditional code doesn't get compiled)
• Small syntax errors in filebuf.h. I got the error error: ‘zstdIn’ was not declared in this scope;

Patch has been tested on zst-compressed FASTQ files. Please comment/merge.

[vejnar]

Fixed in v2.5.0