Linker issue in Homebrew distribution

tl;dr -- v2.1.3 distributed via homebrew is attempting to link against the OpenGL Framework bundle provided in Xcode's `MacOSX10.12.sdk`, which is not guaranteed to exist.

Specifically, the file `/glbinding/2.1.3/cmake/glbinding/glbinding-export.cmake` line 59 is calling `set_target_properties` with

```
INTERFACE_LINK_LIBRARIES "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/OpenGL.framework"
```

On my system (details below) the `MacOSX10.12.sdk` directory doesn't exist. Including glbinding in my own project via a `glbinding_DIR` + `find_package(glbinding REQUIRED)` combination actually resulted in Ninja believing that the OpenGL.Framework was a build target that it couldn't produce.

My Xcode does provide a `MacOSX10.13.sdk` which does contain an OpenGL Framework, but I'm not sure that's what should be targeted. When building from source, glbinding-export.cmake targets `/System/Library/Frameworks/OpenGL.framework` for linking, rather than anything provided by Xcode.

I'm no CMake expert, but my understanding is that these paths are being generated by the `find_package(OpenGL REQUIRED)` call in source/glbinding/CMakeLists.txt. I have no insight into when that call would result in different paths being returned, so I can't offer any suggestions for resolving the mismatch. I also don't know if the Xcode and System OpenGL.Framework bundles differ, so I'm not sure if there should be a preference for one bundle over the other.

As a final confounding note, `/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk` (and presumably `MacOSX10.12.sdk` for systems that have it) is actually a symlink that points to an un-versioned MacOSX.sdk that lives in the same directory. I'm not sure what to do with that information, but it seemed relevant to this discussion.

#### system details

macOS Sierra 10.12.6 (16G1036)

Xcode Version 9.1 (9B55)

cmake version 3.9.5

Homebrew 1.3.6