Prep for Arrow 4.0.0

[ianmcook]

In Arrow 4.0.0 (releasing soon) we made a change to separate the -l and -L flags, putting the former in PKG_LIBS and the latter in PKG_DIRS (https://issues.apache.org/jira/browse/ARROW-11735). This PR updates the boostrapping script accordingly.

[jeroen]

I don't think this makes sense? The entire point of passing -L$BREWDIR is because that is where we extract the libs a few lines above. If you don't pass that path in your PKG_LIBS then the R package won't work?

What are you trying to accomplish with this?

[ianmcook]

The purpose of the changes here is to support changes made in https://github.com/apache/arrow/blob/release-4.0.0/r/configure.

Here are the changes made in r/configure:
apache/arrow@b07027e#diff-089697faebdb7820ca629a2bb316b878cc0ba18a5bfb0b60996f8dbcd1fa11e7

At the bottom of r/configure, notice this:

PKG_LIBS="$PKG_DIRS $PKG_LIBS $BUNDLED_LIBS"

So -L$BREWDIR does get included in PKG_LIBS, but this happens later. During the intermediate steps, we need -L$BREWDIR to be kept separate from the -l flags because that enables us to prepend to them cleanly.

[nealrichardson]

An alternative to this change, if it were preferred, would be to hard-code the extra -DARROW_R_WITH_X flags that get built up in configure: https://github.com/apache/arrow/blob/master/r/configure#L219-L242

Unfortunately, as I understand it, if we don't merge this script change, the mac binaries will build, but they won't have key features enabled (parquet, datasets).