Pass -isysroot to compiler / linker when doing a macOS build

Previously the compiler would use system headers located at
/System/Library/Frameworks, which could result in compilation failures
due to the headers not always being up-to-date in regards to the
latest installed macOS SDK headers that come with Xcode.

Fix the issue by passing the SDK path via the -isysroot option to the
compiler and linker invocations.

If no custom SDK path is given, the build system queries the SDK path
via xcrun --show-sdk-path, which returns something similar to

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/
 /Developer/SDKs/MacOSX.sdk/

Querying via xcrun is now also done for iphone (and simulator)
platforms as well.

Here is an example of a compilation failure message due to outdated
headers:

platform/osx/os_osx.mm:1421:41: error: use of undeclared identifier 'NSAppKitVersionNumber10_12'; did you mean 'NSAppKitVersionNumber'?
                                if (floor(NSAppKitVersionNumber) >= NSAppKitVersionNumber10_12) {
                                                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
                                                                    NSAppKitVersionNumber
/System/Library/Frameworks/AppKit.framework/Headers/NSApplication.h:26:28: note: 'NSAppKitVersionNumber' declared here

1 files changed, 24 insertions, 0 deletions

diff --git a/methods.py b/methods.py
index e9450d95e2..23761b521a 100644
--- a/methods.py
+++ b/methods.py
@@ -613,3 +613,27 @@ def CommandNoCache(env, target, sources, command, **args):
     result = env.Command(target, sources, command, **args)
     env.NoCache(result)
     return result
+
+def detect_darwin_sdk_path(platform, env):
+    sdk_name = ''
+    if platform == 'osx':
+        sdk_name = 'macosx'
+        var_name = 'MACOS_SDK_PATH'
+    elif platform == 'iphone':
+        sdk_name = 'iphoneos'
+        var_name = 'IPHONESDK'
+    elif platform == 'iphonesimulator':
+        sdk_name = 'iphonesimulator'
+        var_name = 'IPHONESDK'
+    else:
+        raise Exception("Invalid platform argument passed to detect_darwin_sdk_path")
+
+    if not env[var_name]:
+        try:
+            sdk_path = subprocess.check_output(['xcrun', '--sdk', sdk_name, '--show-sdk-path']).strip()
+            if sdk_path:
+                env[var_name] = sdk_path
+        except (subprocess.CalledProcessError, OSError) as e:
+            print("Failed to find SDK path while running xcrun --sdk {} --show-sdk-path.".format(sdk_name))
+            raise
+