ARROW-6258: [R] Add macOS build scripts

[nealrichardson]

When installing the R package from source on macOS, if the configure script cannot find libarrow with pkgconfig, and if apache-arrow has not been installed via Homebrew (neither of which is the case on CRAN), an "autobrew" step happens: a script is downloaded and sourced, which uses a fork of Homebrew to download and install binary dependencies for bundling with the R package.

This patch alters the configure script to let you FORCE_AUTOBREW, which is useful for testing, and it will use a local autobrew script file, if found, rather than downloading it. The patch also adds the autobrew script and the apache-arrow.rb brew formula to the r/tools directory, alongside the similar script that downloads the Arrow C++ binary on Windows. The two scripts are copied exactly from their "upstream" versions (noted on L18 of each file), with two minor modifications: (1) autobrew will use a local apache-arrow.rb formula if the file exists, and (2) the formula adds the head reference so you can brew install --build-from-source --HEAD apache-arrow.rb and pull the latest master branch of apache/arrow from GitHub.

See this in action at https://github.com/nealrichardson/arrow-r-nightly/blob/34d27bf482fa1d9f490003a8396fabdff5beea37/.travis.yml. Ordinarily I would add a Travis-CI job to apache/arrow for this, but I know we're anxious not to delay build times further, so I'll run this job nightly. Nightly runs will solve https://issues.apache.org/jira/browse/ARROW-5134, and it will also allow us to host an R package repository with nightly binaries for macOS (and Windows too, using the existing Appveyor config + deploy to bintray).

To install a binary from that repository on macOS, install.packages("arrow", repos="https://dl.bintray.com/nealrichardson/arrow-r").

One TODO: get @jeroen 's approval to include these scripts here under the Apache license and add a citation to LICENSE.txt.

[jeroen]

You have my approval to use these scripts.

[wesm]

Would you mind adding a license file to https://github.com/jeroen/autobrew in case this comes up again in the future?

[nealrichardson]

LICENSE note added. This should be ready to go.

[codecov-io]

Codecov Report

Merging #5095 into master will decrease coverage by 12.38%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #5095       +/-   ##
===========================================
- Coverage   87.64%   75.25%   -12.39%     
===========================================
  Files        1014       57      -957     
  Lines      145922     3617   -142305     
  Branches     1437        0     -1437     
===========================================
- Hits       127887     2722   -125165     
+ Misses      17673      895    -16778     
+ Partials      362        0      -362

Impacted Files	Coverage Δ
python/pyarrow/ipc.pxi
cpp/src/arrow/csv/chunker-test.cc
cpp/src/parquet/column_page.h
cpp/src/parquet/bloom_filter-test.cc
cpp/src/arrow/array/builder_decimal.cc
cpp/src/plasma/client.cc
cpp/src/arrow/io/test-common.h
cpp/src/arrow/util/int-util-test.cc
rust/parquet/src/arrow/converter.rs
cpp/src/arrow/python/io.cc
... and 947 more

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