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Here is an example of a compilation failure message due to outdated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

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