Log Message:
zstd: fix library detection with GNU make 4.3

GNU make 4.3 no longer uses \ to escape # found inside function
invocations, so the \ gets passed through to the printf commands,
causing library detection to fail.

lib/Makefile is patched on pkgsrc by copying detection logic from
programs/Makefile, which has since been updated[0] to support make
4.3 using the compatibility trick suggested in the GNU make changelog.

In particular, since we modify programs/Makefile to link the zstd
binary with the libzstd shared library, failure to detect pthread
in lib/Makefile results in a zstd built with ZSTD_MULTITHREAD to
be linked against a libzstd built without it. This causes "Unsupported
parameter" errors when it is used (except with --single-thread).

So, apply the fix for programs/Makefile to lib/Makefile as well.

[0] https://github.com/facebook/zstd/commit/06a57cf57e3c4e887cadcf688e3081154f3f6db4