Parameterize node_lib_file in addition to node_root_dir

...because on newer Electron versions node-gyp stores the library as
`iojs.lib` and not as `node.lib`.

build.rs

@@ -3,5 +3,6 @@ use std::env;
 fn main() {
     if cfg!(windows) {
         println!("cargo:node_root_dir={}", env::var("DEP_NEON_NODE_ROOT_DIR").unwrap());
+        println!("cargo:node_lib_file={}", env::var("DEP_NEON_NODE_LIB_FILE").unwrap());
     }
 }

crates/neon-sys/build.rs

@@ -48,6 +48,13 @@ fn build_object_file() {
             .expect("Couldn't find node_root_dir in node-gyp output.");
         let node_root_dir_end_index = node_gyp_output[node_root_dir_start_index..].find("'").unwrap() + node_root_dir_start_index;
         println!("cargo:node_root_dir={}", &node_gyp_output[node_root_dir_start_index..node_root_dir_end_index]);
+        let node_lib_file_flag_pattern = "'-Dnode_lib_file=";
+        let node_lib_file_start_index = node_gyp_output
+            .find(node_lib_file_flag_pattern)
+            .map(|i| i + node_lib_file_flag_pattern.len())
+            .expect("Couldn't find node_lib_file in node-gyp output.");
+        let node_lib_file_end_index = node_gyp_output[node_lib_file_start_index..].find(".lib").unwrap() + node_lib_file_start_index;
+        println!("cargo:node_lib_file={}", &node_gyp_output[node_lib_file_start_index..node_lib_file_end_index]);
     }
 
     // Run `node-gyp build` (appending -d in debug mode).