Recent Solaris 11.4 SRUs bundle zstd, but only the 64-bit libraries (no
idea why).  Because of this, in 32-bit builds cc1 etc. fail to link with
undefined references to various ZSTD_* functions from lto-compress.o.
This happens because currently only the presence of <zstd.h> is
necessary to enable zstd support in lto-compress.cc etc.

This patch checks for libzstd first and disables zstd support if
missing.

Tested on sparc-sun-solaris2.11 with the system installation of zstd
(64-bit only) and a locally-compiled one (specified with --with-zstd).

Ok for trunk?

* Setting the cache variable first might be considered a hack:
  alternatively, one could use AC_LINK_IFELSE instead of
  AC_COMPILE_IFELSE, adjusting the test program accordingly.

* All the code withing

  if test "x$with_zstd" != xno; then

  lacks indentation, which makes it harder to read.  I've left that out
  of this patch, though.

	Rainer

-- 
-----------------------------------------------------------------------------
Rainer Orth, Center for Biotechnology, Bielefeld University


2023-03-28  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc:
	* configure.ac (ZSTD_LIB): Move before zstd.h check.
	Unset gcc_cv_header_zstd_h without libzstd.
	* configure: Regenerate.