[PATCH v2] configure: Avoid error messages about missing *-config-*.h files

[Thomas Huth <thuth@redhat.com>]

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>
---
 v2: Use ":>" instead of "touch" as suggested by Philippe

 configure | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/configure b/configure
index 963e35b9a7..bf1c740494 100755
--- a/configure
+++ b/configure
@@ -6458,10 +6458,14 @@ 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
-- 
2.27.0