nixpkgs-tests: add basic test for buildInputs

pkgs/test/stdenv-inputs/lib-main.c

@@ -0,0 +1,14 @@
+#include <stdio.h>
+
+extern unsigned int foo(void);
+extern unsigned int bar(void);
+
+int main(int argc, char **argv)
+{
+  if (foo() != 42)
+    return 1;
+  if (bar() != 42)
+    return 1;
+  fprintf(stderr, "ok\n");
+  return 0;
+}