Upstream decided to remove the cmake scripts for whatever reason.. you can
have a look at the discussion here:
https://github.com/g-truc/glm/issues/947
And check on the status for a new one here:
https://github.com/g-truc/glm/issues/961

This port uses the method the Arch Linux maintainer for glm decided to go.
Big thanks to Sven-Hendrik Haase from my side (see the first link for more details).