unify main entry point regardless of whether linking libc

closes #248

std/bootstrap.zig

@@ -1,17 +1,16 @@
 // This file is in a package which has the root source file exposed as "@root".
+// It is included in the compilation unit when exporting an executable.
 
 const root = @import("@root");
 const std = @import("std");
 
-const want_start_symbol = switch(@compileVar("os")) {
-    Os.linux => true,
-    else => false,
-};
-const want_main_symbol = !want_start_symbol;
+const want_main_symbol = std.build.linkingLibrary("c");
+const want_start_symbol = !want_main_symbol;
 
 const exit = switch(@compileVar("os")) {
     Os.linux => std.linux.exit,
     Os.darwin => std.darwin.exit,
+    else => @compileError("Unsupported OS"),
 };
 
 var argc: usize = undefined;

std/build.zig

@@ -0,0 +1,13 @@
+const mem = @import("mem.zig");
+
+pub fn linkingLibrary(lib_name: []const u8) -> bool {
+    // TODO shouldn't need this if
+    if (@compileVar("link_libs").len != 0) {
+        for (@compileVar("link_libs")) |link_lib| {
+            if (mem.eql(u8, link_lib, lib_name)) {
+                return true;
+            }
+        }
+    }
+    return false;
+}

std/index.zig

@@ -1,3 +1,4 @@
+pub const build = @import("build.zig");
 pub const cstr = @import("cstr.zig");
 pub const debug = @import("debug.zig");
 pub const fmt = @import("fmt.zig");

std/test_runner.zig

@@ -7,7 +7,7 @@ const TestFn = struct {
 
 extern var zig_test_fn_list: []TestFn;
 
-pub fn runTests() -> %void {
+pub fn main(args: [][]u8) -> %void {
     for (zig_test_fn_list) |testFn, i| {
         %%io.stderr.printf("Test {}/{} {}...", i + 1, zig_test_fn_list.len, testFn.name);
 

std/test_runner_libc.zig

@@ -1,6 +0,0 @@
-const test_runner = @import("test_runner.zig");
-
-export fn main(argc: c_int, argv: &&u8) -> c_int {
-    test_runner.runTests() %% return -1;
-    return 0;
-}

std/test_runner_nolibc.zig

@@ -1,5 +0,0 @@
-const test_runner = @import("test_runner.zig");
-
-pub fn main(args: [][]u8) -> %void {
-    return test_runner.runTests();
-}