Add snappy PKGBUILD

[nealrichardson]

In preparation for the upcoming Arrow 0.14 release, on apache/arrow#4622 I worked through building the arrow C++ library for R on Windows from the master branch of apache/arrow. (Aside: I'd like to review that CI setup with you; will follow up separately about that.) In the process, I sorted out a few config changes that were needed due to how the library has evolved in the couple of months since 0.13.

While I got a passing build, I found that the Parquet support that gets built is in practice limited because the library is built without snappy, which is commonly used to compress .parquet files.

> f <- system.file("v0.7.1.parquet", package="arrow")
> library(arrow)
> arrow_available()
[1] TRUE
> df <- read_parquet(f)
Error in read_parquet_file(file) : 
  Arrow error: IOError: Arrow error: NotImplemented: Snappy codec support not built

I supposed that what was needed was to add snappy to this repository and then we could use that as a dependency in the arrow PKGBUILD, so that's what this PR does. This change fails on Appveyor due to another dependency (gcc-libs). Perhaps that needs to be added too, but I'm just learning how all this fits together and figured you could advise whether I'm on the right track before I advanced too much further with trial and error.

[jeroen]

Sorry that this is a bit of a mess.

First of all, for R we only build static libraries. Therefore there exists no mingw-w64-gcc-libs, you can just remove this dependency.

Then, you first need to build your package for https://github.com/r-windows/rtools-packages. In rtools-backports we only build certain libraries that specifically need to be compiled with older compilers. For most packages we use the binaries compiled with the new toolchain.

[jeroen]

Closed via 13ff92a