Removing the “-Werror” from the ToolChains arguments seemed kinda too much to us, so we suppressed only this warning by adding
Result += " -Wno-nonportable-include-path";
to the LinuxToolChain.cs somewhere (below " -Wall -Werror") in GetCLArguments_Global. Note that this is only recommended if you cross-compile. When building for linux on linux, you probably want the build to fail and fix the underlying issue.
(using UE4.20 - v11_clang-5.0.0-centos7)