Fix rc preprocessing when using the MinGW includes and targeting the GNU abi

Also update the standalone test so that this failure would have been detected on any host system.

Author: squeek502

src/Compilation.zig

@@ -4513,6 +4513,19 @@ fn updateWin32Resource(comp: *Compilation, win32_resource: *Win32Resource, win32
             "-fms-compatibility", // Allow things like "header.h" to be resolved relative to the 'root' .rc file, among other things
             "-DRC_INVOKED", // https://learn.microsoft.com/en-us/windows/win32/menurc/predefined-macros
         });
+        // Using -fms-compatibility and targeting the gnu abi interact in a strange way:
+        // - Targeting the GNU abi stops _MSC_VER from being defined
+        // - Passing -fms-compatibility stops __GNUC__ from being defined
+        // Neither being defined is a problem for things like things like MinGW's
+        // vadefs.h, which will fail during preprocessing if neither are defined.
+        // So, when targeting the GNU abi, we need to force __GNUC__ to be defined.
+        //
+        // TODO: This is a workaround that should be removed if possible.
+        if (comp.getTarget().isGnu()) {
+            // This is the same default gnuc version that Clang uses:
+            // https://github.com/llvm/llvm-project/blob/4b5366c9512aa273a5272af1d833961e1ed156e7/clang/lib/Driver/ToolChains/Clang.cpp#L6738
+            try argv.append("-fgnuc-version=4.2.1");
+        }
         for (options.extra_include_paths.items) |extra_include_path| {
             try argv.append("--include-directory");
             try argv.append(extra_include_path);

test/standalone/windows_resources/build.zig

@@ -11,11 +11,14 @@ pub fn build(b: *std.Build) void {
         .abi = .gnu,
     };
 
-    add(b, native_target, test_step);
-    add(b, cross_target, test_step);
+    add(b, native_target, .any, test_step);
+    add(b, cross_target, .any, test_step);
+
+    add(b, native_target, .gnu, test_step);
+    add(b, cross_target, .gnu, test_step);
 }
 
-fn add(b: *std.Build, target: std.zig.CrossTarget, test_step: *std.Build.Step) void {
+fn add(b: *std.Build, target: std.zig.CrossTarget, rc_includes: enum { any, gnu }, test_step: *std.Build.Step) void {
     const exe = b.addExecutable(.{
         .name = "zig_resource_test",
         .root_source_file = .{ .path = "main.zig" },
@@ -26,6 +29,10 @@ fn add(b: *std.Build, target: std.zig.CrossTarget, test_step: *std.Build.Step) v
         .file = .{ .path = "res/zig.rc" },
         .flags = &.{"/c65001"}, // UTF-8 code page
     });
+    exe.rc_includes = switch (rc_includes) {
+        .any => .any,
+        .gnu => .gnu,
+    };
 
     _ = exe.getEmittedBin();