elf2flt: check for presence of libzstd

Binutils-2.40 introduced support for libzstd compressed bfd sections.
By default binutils configure will check for and use the system libzstd
library if it is present.

Now when compiling elf2flt using the bfd library from a
binutils-2.40 and a system present libzstd you will get link errors:

    /usr/bin/ld: ..//binutils-2.40/bfd/libbfd.a(compress.o): in function `decompress_contents':
    ../binutils-2.40/bfd/compress.c:517: undefined reference to `ZSTD_decompress'
    /usr/bin/ld: ../binutils-2.40/bfd/compress.c:519: undefined reference to `ZSTD_isError'
    /usr/bin/ld: ../binutils-2.40/bfd/libbfd.a(compress.o): in function `bfd_compress_section_contents':
    ../binutils-2.40/bfd/compress.c:656: undefined reference to `ZSTD_compress'
    /usr/bin/ld: ../binutils-2.40/bfd/compress.c:661: undefined reference to `ZSTD_isError'

Add a library check to the elf2flt configure for libzstd presence.
When present link using it.

There is no real problem when linking with -lzstd if compiling with
bfd libraries from older binutils, it just won't be needed. So there
is no need to make this check selective on binutils version.

Signed-off-by: Greg Ungerer <gerg@kernel.org>

configure

@@ -3893,6 +3893,51 @@ fi
 else
   LIBS="-L$ac_zlib_prefix/lib -lz $LIBS"
 fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for ZSTD_decompress in -lzstd" >&5
+$as_echo_n "checking for ZSTD_decompress in -lzstd... " >&6; }
+if ${ac_cv_lib_zstd_ZSTD_decompress+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_check_lib_save_LIBS=$LIBS
+LIBS="-lzstd  $LIBS"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char ZSTD_decompress ();
+int
+main ()
+{
+return ZSTD_decompress ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_lib_zstd_ZSTD_decompress=yes
+else
+  ac_cv_lib_zstd_ZSTD_decompress=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+LIBS=$ac_check_lib_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_zstd_ZSTD_decompress" >&5
+$as_echo "$ac_cv_lib_zstd_ZSTD_decompress" >&6; }
+if test "x$ac_cv_lib_zstd_ZSTD_decompress" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_LIBZSTD 1
+_ACEOF
+
+  LIBS="-lzstd $LIBS"
+
+fi
+
 if test "$ac_libiberty" = "NONE"; then
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for objalloc_create in -liberty" >&5
 $as_echo_n "checking for objalloc_create in -liberty... " >&6; }

configure.ac

@@ -124,6 +124,7 @@ if test "$ac_zlib_prefix" = "NONE"; then
 else
   LIBS="-L$ac_zlib_prefix/lib -lz $LIBS"
 fi
+AC_CHECK_LIB(zstd, ZSTD_decompress)
 if test "$ac_libiberty" = "NONE"; then
   AC_CHECK_LIB(iberty, objalloc_create)
   ac_libiberty=auto