Use -fzero-call-used-regs=used on clang 15.

clang 15 seems to have a problem with -fzero-call-used-reg=all which
causes spurious "incorrect signature" failures with ED25519.  On those
versions, use -fzero-call-used-regs=used instead.  (We may add exceptions
later if specific versions prove to be OK).  Also move the GCC version
check to match.

Initial investigation by Daniel Pouzzner (douzzer at mega nu), workaround
suggested by Bill Wendling (morbo at google com).  bz#3475, ok djm@

configure.ac

@@ -172,6 +172,22 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[int main(void) { return 0; }]])],
 CFLAGS="$saved_CFLAGS"
 
 if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
+	AC_MSG_CHECKING([gcc version])
+	GCC_VER=`$CC -v 2>&1 | $AWK '/gcc version /{print $3}'`
+	case "$GCC_VER" in
+		1.*) no_attrib_nonnull=1 ;;
+		2.8* | 2.9*)
+		     no_attrib_nonnull=1
+		     ;;
+		2.*) no_attrib_nonnull=1 ;;
+		*) ;;
+	esac
+	AC_MSG_RESULT([$GCC_VER])
+
+	AC_MSG_CHECKING([clang version])
+	CLANG_VER=`$CC -v 2>&1 | $AWK '/clang version /{print $3}'`
+	AC_MSG_RESULT([$CLANG_VER])
+
 	OSSH_CHECK_CFLAG_COMPILE([-pipe])
 	OSSH_CHECK_CFLAG_COMPILE([-Wunknown-warning-option])
 	OSSH_CHECK_CFLAG_COMPILE([-Wno-error=format-truncation])
@@ -203,20 +219,15 @@ if test "$GCC" = "yes" || test "$GCC" = "egcs"; then
 	# actually links. The test program compiled/linked includes a number
 	# of integer operations that should exercise this.
 	OSSH_CHECK_CFLAG_LINK([-ftrapv])
-	OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=all])
+	# clang 15 seems to have a big in -fzero-call-used-regs=all.  See
+	# https://bugzilla.mindrot.org/show_bug.cgi?id=3475 and
+	# https://github.com/llvm/llvm-project/issues/59242
+	case "$CLANG_VER" in
+	15.*) OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=used]) ;;
+	*)    OSSH_CHECK_CFLAG_COMPILE([-fzero-call-used-regs=all]) ;;
+	esac
 	OSSH_CHECK_CFLAG_COMPILE([-ftrivial-auto-var-init=zero])
     fi
-	AC_MSG_CHECKING([gcc version])
-	GCC_VER=`$CC -v 2>&1 | $AWK '/gcc version /{print $3}'`
-	case $GCC_VER in
-		1.*) no_attrib_nonnull=1 ;;
-		2.8* | 2.9*)
-		     no_attrib_nonnull=1
-		     ;;
-		2.*) no_attrib_nonnull=1 ;;
-		*) ;;
-	esac
-	AC_MSG_RESULT([$GCC_VER])
 
 	AC_MSG_CHECKING([if $CC accepts -fno-builtin-memset])
 	saved_CFLAGS="$CFLAGS"