c: move c files into nostrdb in prep for switchover

nostrdb/build_assert.h

@@ -0,0 +1,40 @@
+/* CC0 (Public domain) - see LICENSE file for details */
+#ifndef CCAN_BUILD_ASSERT_H
+#define CCAN_BUILD_ASSERT_H
+
+/**
+ * BUILD_ASSERT - assert a build-time dependency.
+ * @cond: the compile-time condition which must be true.
+ *
+ * Your compile will fail if the condition isn't true, or can't be evaluated
+ * by the compiler.  This can only be used within a function.
+ *
+ * Example:
+ *    #include <stddef.h>
+ *    ...
+ *    static char *foo_to_char(struct foo *foo)
+ *    {
+ *        // This code needs string to be at start of foo.
+ *        BUILD_ASSERT(offsetof(struct foo, string) == 0);
+ *        return (char *)foo;
+ *    }
+ */
+#define BUILD_ASSERT(cond) \
+    do { (void) sizeof(char [1 - 2*!(cond)]); } while(0)
+
+/**
+ * BUILD_ASSERT_OR_ZERO - assert a build-time dependency, as an expression.
+ * @cond: the compile-time condition which must be true.
+ *
+ * Your compile will fail if the condition isn't true, or can't be evaluated
+ * by the compiler.  This can be used in an expression: its value is "0".
+ *
+ * Example:
+ *    #define foo_to_char(foo)                    \
+ *         ((char *)(foo)                        \
+ *          + BUILD_ASSERT_OR_ZERO(offsetof(struct foo, string) == 0))
+ */
+#define BUILD_ASSERT_OR_ZERO(cond) \
+    (sizeof(char [1 - 2*!(cond)]) - 1)
+
+#endif /* CCAN_BUILD_ASSERT_H */