If I had to guess, are you using R 3.4.0+? Note that there's new Clang and Fortran environment requirements, which may not be native to your machine and cause Rcpp packages (like motifmatchr) to fail.

The guide from CRAN is pretty good/clear/straightforward about this--
https://cloud.r-project.org/bin/macosx/tools/

(This personally solved my issue installing chromVAR for Mac for R 3.4)

An alternative--

brew install cmake gcc

which in the latest version, should install gcc-7. You can use this instead of clang. To do so for installing mac R packages, modify the CC and CXX variables in Makevars like so:

lareauc$ head -2 ~/.R/Makevars 
CC=/usr/local/bin/gcc-7 
CXX=/usr/local/bin/g++-7 

Installed!!! Thanks a lot.