configure: Avoid error messages about missing *-config-*.h files

When compiling with --disable-system there is a harmless yet still
annoying error message at the end of the "configure" step:

 sed: can't read *-config-devices.h: No such file or directory

When only building the tools or docs, without any emulator at all,
there is even an additional message about missing *-config-target.h
files.

Fix it by checking whether any of these files are available before
using them.

Fixes: e0447a834d ("configure: Poison all current target-specific #defines")
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210519113840.298174-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -6503,10 +6503,14 @@ fi
 
 # Create list of config switches that should be poisoned in common code...
 # but filter out CONFIG_TCG and CONFIG_USER_ONLY which are special.
-sed -n -e '/CONFIG_TCG/d' -e '/CONFIG_USER_ONLY/d' \
-    -e '/^#define / { s///; s/ .*//; s/^/#pragma GCC poison /p; }' \
-    *-config-devices.h *-config-target.h | \
-    sort -u > config-poison.h
+target_configs_h=$(ls *-config-devices.h *-config-target.h 2>/dev/null)
+if test -n "$target_configs_h" ; then
+    sed -n -e '/CONFIG_TCG/d' -e '/CONFIG_USER_ONLY/d' \
+        -e '/^#define / { s///; s/ .*//; s/^/#pragma GCC poison /p; }' \
+        $target_configs_h | sort -u > config-poison.h
+else
+    :> config-poison.h
+fi
 
 # Save the configure command line for later reuse.
 cat <<EOD >config.status