build: fix linking to dev SDK on macOS

on macOS mpv was linked to the system SDK which didn't cause any
problems as long as the system SDK was the same as the dev SDK. though
it started to cause linking warnings when a new xcode version with the
SDK for the next macOS was installed. in the worst case it could also
cause linking errors. to fix this we explicitly set the SDK path to the
SDK that is used for building instead.

waftools/checks/custom.py

@@ -92,6 +92,18 @@ def fn(ctx, dependency_identifier):
         return ret
     return fn(ctx, dependency_identifier)
 
+def _run(cmd):
+    from waflib import Utils
+    try:
+        cmd = Utils.subprocess.Popen(cmd,
+                                     stdout=Utils.subprocess.PIPE,
+                                     stderr=Utils.subprocess.PIPE,
+                                     shell=True)
+        output = cmd.stdout.read().strip()
+        return output
+    except Exception:
+        return ""
+
 def check_cocoa(ctx, dependency_identifier):
     fn = check_cc(
         fragment         = load_fragment('cocoa.m'),
@@ -100,7 +112,15 @@ def check_cocoa(ctx, dependency_identifier):
         includes         = ctx.srcnode.abspath(),
         linkflags        = '-fobjc-arc')
 
-    return fn(ctx, dependency_identifier)
+    res = fn(ctx, dependency_identifier)
+    # on macOS we explicitly need to set the SDK path, otherwise it can lead to
+    # linking warnings or errors
+    if res:
+        ctx.env.append_value('LINKFLAGS', [
+            '-isysroot', '%s' % _run('xcrun --sdk macosx --show-sdk-path')
+        ])
+
+    return res
 
 def check_openal(ctx, dependency_identifier):
     checks = [