Android NDK starting from r18 has libcxx, and it's the only STL available. (reference: https://developer.android.com/ndk/guides/cpp-support#cs)
r18 is ~6 years old at this point.
This check dates to ~8 years ago (predating r18) and was important at that time.
Right now, it can be clearly stated that given the C++11 requirement stated in README for GLM, as well as (unless you are building with very outdated toolchain) - all modern Android is built with NDK toolchain that is newer than r18 - this check can be removed, and all the functionality can by default delegate to general detection for STL compatibility.
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