Add option to enable OpenMP

[coatless]

I've been seeing an uptick in e-mails and questions related to OpenMP on mac. The thought right now is to check the Apple clang string and, then, download the relevant tar, extract the contents, and modify ~/.Rprofile.

We'll probably add it as an extra option to "opt-in" to.

Relevant tars & mappings are available on https://mac.r-project.org/openmp/

table of LLVM release & headers

Build	Download	SHA1 checksum
LLVM 14.0.6Xcode 14+ (Apple clang 1400.x)	openmp-14.0.6-darwin20-Release.tar.gz (Release)openmp-14.0.6-darwin20-Debug.tar.gz (Debug)	19912991431ecf032f037b6e8aea19dbd490f1ba6b96a15db9329ea6f605449a630575036fa20aae
LLVM 13.0.0Xcode 13.3-13.4.1 (Apple clang 1316.x)	openmp-13.0.0-darwin21-Release.tar.gz (Release)openmp-13.0.0-darwin21-Debug.tar.gz (Debug)	47af4cb0d1f3554969f2ec9dee450d728ea30024f6f8f1f49c02d5ec0729b56ddc7eaf51e7f04714
LLVM 12.0.1Xcode 13.0-13.2.1 (Apple clang 1300.x)	openmp-12.0.1-darwin20-Release.tar.gz (Release)openmp-12.0.1-darwin20-Debug.tar.gz (Debug)	4fab53ccc420ab882119256470af15c210d19e5e58b4323e7933e12cba5c2996b4c9ef27567c41d9
LLVM 11.0.1 (+M1 patch)Xcode 12.5 (Apple clang 1205.x)	openmp-11.0.1-darwin20-Release.tar.gz (Release)openmp-11.0.1-darwin20-Debug.tar.gz (Debug)	0dcd19042f01c4f552914e2cf7a53186de397aa165e83ea667c72bbe44fea699776564d2f03a080f

(All binaries above include both arm64 and x86_64 and require macOS 11 or higher. Binaries below require macOS 10.13 or higher and are Intel-only)

Build	Download	SHA1 checksum
LLVM 10.0.0Xcode 12.0-12.4 (Apple clang 1200.x)	openmp-10.0.0-darwin17-Release.tar.gz (Release)openmp-10.0.0-darwin17-Debug.tar.gz (Debug)	9bf16a64ab747528c5de7005a1ea1a9e318b3cf0d4508d3f0c2952c3f984393b088e0b4beab33b58
LLVM 9.0.1Xcode 11.4-11.7 (Apple clang 1103.x)	openmp-9.0.1-darwin17-Release.tar.gz (Release)openmp-9.0.1-darwin17-Debug.tar.gz (Debug)	e5bd8501a3f957b4babe27b0a266d4fa15dbc23fc4c8491631504fb060f7c25ec14324d02d617d5b
LLVM 8.0.1Xcode 11.0-11.3.1 (Apple clang 1100.x)	openmp-8.0.1-darwin17-Release.tar.gz (Release)openmp-8.0.1-darwin17-Debug.tar.gz (Debug)	e4612bfcb1bf520bf22844f7db764cadb7577c28d6c83918b28405d43950d4b864ca8d1687eed4d1
LLVM 7.1.0Xcode 10.2-10.3 (Apple clang 1001.x)	openmp-7.1.0-darwin17-Release.tar.gz (Release)openmp-7.1.0-darwin17-Debug.tar.gz (Debug)	6891ff6f83f2ed83eeed42160de819b50cf643cd34456adde62b9a1047f906e1d7f54990a1c15a34