Address Header Change on macOS 10.14 (Mojave)

[coatless]

Incorporate in the headless Xcode CLI install for Mojave.

sudo installer -pkg \
/Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg \
-target /

Avoid messing with Makevars via:

sudo touch ~/.R/Makevars && sudo echo 'CPPFLAGS="-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include"' >> ~/.R/Makevars

c.f. https://twitter.com/axiomsofxyz/status/1045463113863180290, https://cran.r-project.org/doc/manuals/r-patched/R-admin.html#macOS

[coatless]

Download procedure needs to also switch over to:

sudo xcode-select --reset
sudo xcode-select --install

We can no longer just use:

sudo xcode-select --install

h/t @jimhester, c.f. https://stackoverflow.com/a/53089500/2055486

[coatless]

Done inside of #19.