tests: c_lib: test for declaration of signal()

We don't actually implement signal() in Zephyr, but some users
require the declaration.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

tests/lib/c_lib/src/test_signal_decl.c

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2023 Meta
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <signal.h>
+
+#include <zephyr/ztest.h>
+
+ZTEST(test_c_lib, test_signal_decl)
+{
+ /*
+ * Even though the use of the ANSI-C function "signal()" is not yet
+ * supported in Zephyr, some users depend on it being declared (i.e.
+ * without CONFIG_POSIX_API=y and with with CONFIG_POSIX_SIGNAL=n).
+ */
+
+ zassert_not_null(signal);
+}