Issue title: Segmentation fault when building on windows?
 Issue body: Hi,

I'm trying to build libigl in windows 10 and Visual Studio 2015.
From the `libigl` root folder I simply run

```
mkdir build && cd build
cmake ../ -G "Visual Studio 14 2015 Win64"
```

The output I get is:

```
-- Selecting Windows SDK version  to target Windows 10.0.15063.
-- The C compiler identification is MSVC 19.0.24215.1
-- The CXX compiler identification is MSVC 19.0.24215.1
-- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe
-- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- MSVC -> forcing use of statically-linked runtime.
-- Looking for pthread.h
-- Looking for pthread.h - not found
-- Found Threads: TRUE
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5ore (igl)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5omiso (igl_comiso)
-- Found Git: C:/Program Files/Git/mingw64/bin/git.exe (found version "2.25.1.windows.1")
CMake Warning (dev) at external/embree/common/cmake/test.cmake:31 (SET):
  implicitly converting 'INT' to 'STRING' type.
Call Stack (most recent call first):
  external/embree/CMakeLists.txt:101 (INCLUDE)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5mbree (igl_embree)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5opengl (igl_opengl)
-- Found OpenGL: opengl32
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5opengl_glfw (igl_opengl_glfw)
-- Using Win32 for window creation
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5opengl_glfw_imgui (igl_opengl_glfw_imgui)
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Failed
-- Performing Test COMPILER_HAS_DEPRECATED
-- Performing Test COMPILER_HAS_DEPRECATED - Success
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5png (igl_png)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5tetgen (igl_tetgen)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5triangle (igl_triangle)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5predicates (igl_predicates)
-- Creating target: igl38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5xml (igl_xml)
-- Found PythonInterp: C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python36_64/python.exe (found suitable version "3.6.6", minimum required is "3.4")
-- Found PythonLibs: C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python36_64/libs/python36.lib (found suitable version "3.6.6", minimum required is "3.4")
-- Found PythonInterp: C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python36_64/python.exe (found version "3.6.6")
-- Found PythonLibs: C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python36_64/libs/Python36.lib
-- pybind11 v2.3.dev0
-- Performing Test HAS_MSVC_GL_LTCG
-- Performing Test HAS_MSVC_GL_LTCG - Success
-- LTO enabled
-- Configuring done
Segmentation fault
```

Any clue on why I'm getting a segmentation fault?
I've also tried Visual Studio 2017, the result is the same.
...

#### Check all that apply (change to `[x]`)
- [x] Windows
- [ ] Mac OS X
- [ ] Linux
- [ ] I have tried the `dev` branch and the problem persists

See https://libigl.github.io/CONTRIBUTING/#bugreport for more tips.

 Comments: 
 Comment 0: Hi, please try the latest `dev` branch and let us know if the problem still persists.
 Comment 1: Just tried, same error. Can you help?
 Comment 2: Seems that you are trying to run CMake through mingw or smth like that. Please try to generate a VS project via CMake GUI instead.
 Comment 3: I can't see why it would be different but I'll give it s go
 Comment 4: That didn't work either. However I was using cmake 3.17, downgrading to the previous release worked instead. (cmake 3.16).
 Comment 5: CMake 3.17 is not a stable release yet. I would wait until they release a stable version and double-check again if the compilation is still failing.
{'source': 'GithubIssues', 'repo_name_with_owner': 'libigl/libigl', 'issue_id': 578903476, 'repo_stars': 3646.0}