I had the same problem as tjl, and I used AntoineC's code to force R to accept the rtools version.
find_rtool() returns TRUE now, but when I'm trying to install transformr (devtools::install_github("thomasp85/transformr") it's still asking me to download further tools:
When I chose yes, I'm getting the following error message:
Installation failed: Could not find build tools necessary to build transformr
I am using R version 3.5.1, I have the newest version of rtools (downloaded today) and already tried reinstalling it.
Am I missing something? Does anyone still have this problem despite using the code that AntoineC posted?