fixes for compiling with cmake generated Xcode project

[AndySomogyi]

Hi,

I added a few fixes that enables compilation of nanogui with CMake generated Xcode projects. Xcode has a bug, and I've implemented the CMake suggested work around.

I've verified that this fix works for both Xcode generated projects, and CMake generated makefiles.

[AndySomogyi]

Hi,

looks like something strange is going on with the Travis CI, as the nanogui CMakeLists.txt has required CMake version 2.8.12 for some time, but for some reason, the host operating system seems to have been downgraded to version 2.8.7.

Output from Travis CI on the gcc 4.8 test:
CMake 2.8.12 or higher is required. You are running version 2.8.7

[wjakob]

Apologies for the big delay. Two issue that I see with the PR are

1. It fails the CI tests (probably easily fixable by rebasing)
2. It hacks the STB implementation exports to across the platforms, rather than switching to a uniform scheme. What would have made much more sense to me is to ensure that the symbols are exported on all platforms, and then import them consistently.

Items #1 and #2 are really independent from each other. For that reason, it may be easier to split this into two separate PRs.

Thanks,
Wenzel

[svenevs]

The Travis builds failed because this was submitted before #224. If you close and reopen the PR that will relaunch Travis.