build failure with Xcode 10 - libstdc++ not supported anymore
#### Code Sample, a copy-pastable example if possible

```python
(pandas-dev) bilbo:pandas-rb robertbuckley$ python setup.py build_ext --inplace -j 4

running build_ext
building 'pandas._libs.window' extension
gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/robertbuckley/anaconda3/envs/pandas-dev/include -arch x86_64 -I/Users/robertbuckley/anaconda3/envs/pandas-dev/include -arch x86_64 -Ipandas/_libs -I./pandas/_libs -Ipandas/_libs/src/klib -Ipandas/_libs/src -I/Users/robertbuckley/anaconda3/envs/pandas-dev/lib/python3.6/site-packages/numpy/core/include -I/Users/robertbuckley/anaconda3/envs/pandas-dev/include/python3.6m -c pandas/_libs/window.cpp -o build/temp.macosx-10.7-x86_64-3.6/pandas/_libs/window.o -Wno-unused-function
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]
pandas/_libs/window.cpp:655:10: fatal error: 'ios' file not found
#include "ios"
         ^~~~~
1 warning and 1 error generated.
error: command 'gcc' failed with exit status 1
```
#### Problem description

I setup a dev environment on Mac OS 10.13 according to https://pandas.pydata.org/pandas-docs/stable/contributing.html#creating-a-development-environment and all was well

Upgraded to 10.14 (mojave) and Xcode 10, and now i get the above failure.

[Xcode 10 release notes](https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes?language=objc) includes the following deprecation notice

> 
Building with libstdc++ was deprecated with Xcode 8 and is not supported in Xcode 10 when targeting iOS. C++ projects must now migrate to libc++ and are recommended to set a deployment target of macOS 10.9 or later, or iOS 7 or later. Besides changing the C++ Standard Library build setting, developers should audit hard-coded linker flags and target dependencies to remove references to libstdc++ (including -lstdc++, -lstdc++.6.0.9, libstdc++.6.0.9.tbd, and libstdc++.6.0.9.dylib). Project dependencies such as static archives that were built against libstdc++ will also need to be rebuilt against libc++. (40885260)

Some workaround suggestions are at https://stackoverflow.com/questions/52425766/stdlibc-headers-not-found-error-on-xcode-10 and the answer its linked to. 

I dont have access to Xcode 9 anymore, so i can't try the workaround of copying the old libs from there