Multi-file BPF C builds (#2393)

programs/bpf/c/src/struct_ret/struct_ret.c

@@ -0,0 +1,18 @@
+#include <solana_sdk.h>
+
+struct foo {const uint8_t *input;};
+struct foo bar(const uint8_t *input);
+
+extern bool entrypoint(const uint8_t *input) {
+  struct foo foo = bar(input);
+  sol_log_64(0, 0, 0, (uint64_t)input, (uint64_t)foo.input);
+  sol_assert(input == foo.input);
+  return true;
+}
+
+struct foo bar(const uint8_t *input) {
+  struct foo foo;
+  foo.input = input;
+  return foo;
+}
+