asv config to use `build_ext` instead of `build`
Not 100% sure on this change, feel free to close at any time.

---

I have not seen a single mention in the docs of ```python setup.py build``` there was only mention of ```python setup.py build_ext```, so I thought that asv should follow that as well (I could be wrong here).

Not sure of the differences between all of these. Is there anything documented in distutils to explain?

FWIW I think the asv command is pulled directly from the ASV docs @WillAyd I have tried to look for the past hour for the differences, could not find anything that would clarify the differences.

---

I did look at the source of each command (```build``` and ```build_ext```) and it looks like ```build``` is also executing the commands ```build_ext``` as well as ```build_py```, ```build_clib``` and ```build_scripts```. I could not understand from the docs/source code what each of them are doing. so maybe better to not touch this one as I don't see a clear difference when using ```build``` over ```build_ext```, one thing I did noticed is that when running ```python setup.py build``` it copies a lot of files from the ```pandas/tests/``` directory into the ```build/``` directory.


Those are just __some__ of the files, there are *many* more.

<details>

```
copying pandas/tests/io/parser/__init__.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/conftest.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_c_parser_only.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_comment.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_common.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_compression.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_converters.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_dialect.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_dtypes.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_encoding.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_header.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_index_col.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_mangle_dupes.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_multi_thread.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_na_values.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_network.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_parse_dates.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_python_parser_only.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_quoting.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_read_fwf.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_skiprows.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_textreader.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_unsupported.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
copying pandas/tests/io/parser/test_usecols.py -> build/lib.linux-x86_64-3.7/pandas/tests/io/parser
```

</details>

---

@WillAyd I think you should close this PR, as I can't really tell the difference between the two commands. Sounds good. If there's no benefit to changing let's just leave for now