macho: handle -install_name option for dylibs/MachO

The status quo for the `build.zig` build system is preserved in
the sense that, if the user does not explicitly override
`dylib.setInstallName(...);` in their build script, the default
of `@rpath/libname.dylib` applies. However, should they want to
override the default behaviour, they can either:

1) unset it with

```dylib.setIntallName(null);```

2) set it to an explicit string with

```dylib.setInstallName("somename.dylib");```

When it comes to the command line however, the default is not to
use `@rpath` for the install name when creating a dylib. The user
will now be required to explicitly specify the `@rpath` as part
of the desired install name should they choose so like so:

1) with `build-lib`

```
zig build-lib -dynamic foo.zig -install_name @rpath/libfoo.dylib
```

2) with `cc`

```
zig cc -shared foo.c -o libfoo.dylib -Wl,"-install_name=@rpath/libfoo.dylib"
```

1 files changed, 3 insertions, 0 deletions

diff --git a/src/Compilation.zig b/src/Compilation.zig
index b3c2608c05..861771def4 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -780,6 +780,8 @@ pub const InitOptions = struct {
     enable_link_snapshots: bool = false,
     /// (Darwin) Path and version of the native SDK if detected.
     native_darwin_sdk: ?std.zig.system.darwin.DarwinSDK = null,
+    /// (Darwin) Install name of the dylib
+    install_name: ?[]const u8 = null,
 };
 
 fn addPackageTableToCacheHash(
@@ -1509,6 +1511,7 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
             .use_stage1 = use_stage1,
             .enable_link_snapshots = options.enable_link_snapshots,
             .native_darwin_sdk = options.native_darwin_sdk,
+            .install_name = options.install_name,
         });
         errdefer bin_file.destroy();
         comp.* = .{