add swift as main dependency so all dependencies can be disabled easily

waftools/checks/custom.py

@@ -115,6 +115,8 @@ def check_cocoa(ctx, dependency_identifier):
 def check_swift(ctx, dependency_identifier):
     if ctx.env.SWIFT_VERSION:
         major = int(ctx.env.SWIFT_VERSION.split('.')[0])
+        ctx.add_optional_message(dependency_identifier,
+                                 'version found: ' + ctx.env.SWIFT_VERSION)
         if major >= 3:
             return True
     return False

wscript

@@ -170,6 +170,11 @@ main_dependencies = [
         'req': True,
         'fmsg': 'Unable to find either POSIX or MinGW-w64 environment, ' \
                 'or compiler does not work.',
+    }, {
+        'name': '--swift',
+        'desc': 'macOS Swift build tools',
+        'deps': 'os-darwin',
+        'func': check_swift,
     }, {
         'name': '--uwp',
         'desc': 'Universal Windows Platform',
@@ -914,8 +919,8 @@ standalone_features = [
      }, {
         'name': '--macos-cocoa-cb',
         'desc': 'macOS opengl-cb backend',
-        'deps': 'cocoa',
-        'func': check_swift
+        'deps': 'cocoa  && swift',
+        'func': check_true
     }
 ]