[PATCH] build: Silence clang warning on older glib autoptr usage

[Eric Blake <eblake@redhat.com>]

glib's G_DEFINE_AUTOPTR_CLEANUP_FUNC() macro defines several static
inline functions, often with some of them unused, but prior to 2.57.2
did not mark the functions as such.  As a result, clang (but not gcc)
fails to build with older glib unless -Wno-unused-function is enabled.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eric Blake <eblake@redhat.com>
---

Half-tested: I proved to myself that this does NOT enable
-Wno-unused-function on my setup of glib 2.62.5 and gcc 9.2.1 (Fedora
31), but would do so if I introduced an intentional compile error into
the sample program; but Iwas unable to test that it would prevent the
build failure encountered by Peter on John's pull request (older glib
but exact version unknown, clang, on NetBSD).

 configure | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/configure b/configure
index eb49bb6680c1..57a72f120aa9 100755
--- a/configure
+++ b/configure
@@ -3832,6 +3832,26 @@ if ! compile_prog "$glib_cflags -Werror" "$glib_libs" ; then
     fi
 fi

+# Silence clang warnings triggered by glib < 2.57.2
+cat > $TMPC << EOF
+#include <glib.h>
+typedef struct Foo {
+    int i;
+} Foo;
+static void foo_free(Foo *f)
+{
+    g_free(f);
+}
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(Foo, foo_free);
+int main(void) { return 0; }
+EOF
+if ! compile_prog "$glib_cflags -Werror" "$glib_libs" ; then
+    if cc_has_warning_flag "-Wno-unused-function"; then
+        glib_cflags="$glib_cflags -Wno-unused-function"
+        CFLAGS="$CFLAGS -Wno-unused-function"
+    fi
+fi
+
 #########################################
 # zlib check

-- 
2.25.1