Build fix: Turn off _GLIBCXX_DEBUG based on a compile check

Summary:
Enabling _GLIBCXX_DEBUG (implied by LLVM_ENABLE_EXPENSIVE_CHECKS) causes
std::min_element (and presumably others) to no longer be constexpr, which
in turn causes the build to fail.

This seems like a bug in the GCC STL. This change works around it.

Change-Id: I5fc471caa9c4de3ef4e87aeeac8df1b960e8e72c

Reviewers: tstellar, hans, serge-sans-paille

Differential Revision: https://reviews.llvm.org/D75199