That is because clang picks up the system GCC 4.8.5 in the second build, which doesn’t support fully C++14. Unfortunately in the past you can set GCC_INSTALL_PREFIX when building LLVM, but it is removed recently (Add --gcc-install-dir=, deprecate --gcc-toolchain=, and remove GCC_INSTALL_PREFIX). CCC_OVERRIDE_OPTIONS seems the only way to tell clang to use your own GCC. You can check clang/tools/driver/driver.cpp line 105 to see how to use that environment variable.
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