package/zstd: fix dynamic build without threads

package/zstd: fix dynamic build without threads

Dynamic build without threads is broken since bump to version 1.5.0 in
commit aad8bbf58842dd62a4911d2c671ad9598e26b459

Indeed, dynamic library is built with threads support since
https://github.com/facebook/zstd/commit/91465e23b2710de031a762874cafb417f8b7556e

To fix this build failure, add -nomt if needed however libzstd-nomt is a
"special target that builds a library in single-thread mode _and_
without zstdmt_compress.c". For an unknown reason, this target fails to
build and don't create any symlinks so replace libzstd-{mt,nomt} by
lib-{mt,nomt} even if this will have the side effect of building a
shared and a static version of the library.

Move the existing HAVE_THREAD setting in the same condition.

While at it, also replace "libzstd.a libzstd" by "lib"

Fixes:
 - http://autobuild.buildroot.org/results/e609601a0fc91c44d88a12c35b29ce937381462f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Arnout: move HAVE_THREAD in the same condition]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>