Use -isysroot on Catalina too, not just Big Sur

[mk12]

I am writing a Zig program on macOS Catalina 10.15.7 that uses system frameworks.

Under Zig 0.7.1, I had to use this workaround otherwise it wouldn't find framework headers such as #include <Carbon/Carbon.h>:

fn getMacFrameworksDir(b: *Builder) ![]u8 {
    const sdk = try b.exec(&[_][]const u8{ "xcrun", "--show-sdk-path" });
    const parts = &[_][]const u8{
        std.mem.trimRight(u8, sdk, "\n"),
        "/System/Library/Frameworks",
    };
    return std.mem.concat(b.allocator, u8, parts);
}

pub fn build(b: *Builder) !void {
    // ...
    exe.addFrameworkDir(try getMacFrameworksDir(b));
    // ...
}

I then switched to tip-of-tree Zig (8661a13, zig version is 0.8.0-dev.1118+8661a13b7) which I built from source using Option A, and it stopped working — with or without my workaround. However, after applying this patch to make main.zig use -isysroot on Catalina or later, it worked, and I don't need the workaround anymore.

This was already done in #7506, but only for Big Sur or later. This PR just expands the check to include Catalina.

(Or, maybe we should just always attempt getSDKPath on macOS? Threads complaining about these headers moving go back years, so I'm not sure if Catalina is the earliest where it can happen.)

Related issues: #7697, #2208

[mikdusan]

Big Sur forced zig to use xcrun --show-sdk-path because no headers or linking was possible without it. Technically speaking, if memory serves, Catalina was the first macOS to ship without /usr/include but it still had system .dylib available in the normal filesystem. And some developers forced cmdlinetools to install headers with a manual switch but it was not recommended.

Thus I'm inclined to include Catalina in automatic SDK detection and use.

[mikdusan]

looks good. Thanks!