[PATCH] build: Check that -lzstd can be linked

[Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>]

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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.


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diff --git a/gcc/configure.ac b/gcc/configure.ac
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -1494,6 +1494,14 @@ fi
 CXXFLAGS="$CXXFLAGS $ZSTD_CPPFLAGS"
 LDFLAGS="$LDFLAGS $ZSTD_LDFLAGS"
 
+# LTO can use zstd compression algorithm
+save_LIBS="$LIBS"
+LIBS=
+AC_SEARCH_LIBS(ZSTD_compress, zstd,, gcc_cv_header_zstd_h=no)
+ZSTD_LIB="$LIBS"
+LIBS="$save_LIBS"
+AC_SUBST(ZSTD_LIB)
+
 AC_MSG_CHECKING(for zstd.h)
 AC_CACHE_VAL(gcc_cv_header_zstd_h,
 # We require version 1.3.0 or later.  This is the first version that has
@@ -1512,14 +1520,6 @@ if test $gcc_cv_header_zstd_h = yes; the
 elif test "x$with_zstd" != x; then
     as_fn_error $? "Unable to find zstd.h.  See config.log for details." "$LINENO" 5
 fi
-
-# LTO can use zstd compression algorithm
-save_LIBS="$LIBS"
-LIBS=
-AC_SEARCH_LIBS(ZSTD_compress, zstd)
-ZSTD_LIB="$LIBS"
-LIBS="$save_LIBS"
-AC_SUBST(ZSTD_LIB)
 fi
 
 dnl Disabled until we have a complete test for buggy enum bitfields.