I want to provide what I did to solve this per the comments of @Tsyvarev and @drescherjm

I downloaded the actual source from the GLM releases page

After unzipping the source, at the root of the source, I ran the commands:

C:\cmake\bin\cmake -DGLM_BUILD_TESTS=OFF -DBUILD_SHARED_LIBS=OFF -B build . -G "MinGW Makefiles"
C:\cmake\bin\cmake --build build -- all
C:\cmake\bin\cmake --build build -- install

Adding "MinGW Makefiles" resolved the "compiler variable not set" errors I was getting and since the source contained the GLM_BUILD_INSTALL, the install executed without errors.