pip install annoy not working on macOS mojave

This is where `pip install annoy` fails:



```bash

  gcc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/Users/taegyung/miniconda3/include -arch x86_64 -I/Users/taegyung/miniconda3/include -arch x86_64 -I/Users/taegyung/miniconda3/include/python3.6m -c src/annoymodule.cc -o build/temp.macosx-10.7-x86_64-3.6/src/annoymodule.o -O3 -ffast-math -fno-associative-math -march=native

  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]

  In file included from src/annoymodule.cc:15:

  src/annoylib.h:49:10: fatal error: 'vector' file not found

  #include <vector>

           ^~~~~~~~

  1 warning and 1 error generated.

  error: command 'gcc' failed with exit status 1

```



I tried some things I found from previous issues and some general solutions accros the web, including



- sudo pip install annoy

- Installing XCode 9.4 (I wasn't using any version of XCode previously)

- xcode-select --install

- brew remove gcc; brew install gcc



I finally got it by running `conda install -c akode annoy`, but I'm not sure if this is the right way to do it (and don't know why this even works.)

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