hi, i am using gentoo linux with latest mono 5.18.1.0 and clang 8.0.0
after downloading 32 gb of dependencies via your setup.sh script i am very disappointed to discover this message:
./GenerateProjectFiles.sh
Setting up Unreal Engine 4 project files…
- xbuild Source/Programs/UnrealBuildTool/UnrealBuildTool.csproj /verbosity:quiet /nologo /p:TargetFrameworkVersion=v4.5 /p:Configuration=Development
- mono Binaries/DotNET/UnrealBuildTool.exe -makefile -kdevelopfile -qmakefile -cmakefile -codelitefile
Generating data for project indexing… 0%
UnrealBuildTool Exception: ERROR: This version of the Unreal Engine can only be compiled with clang 3.9, 3.8, 3.7, 3.6 and 3.5. clang 8.0.0 may not build it - please use a different version.
please update your unrealtournament and unrealengine git repositorys at github to use a recent LOCAL clang 8.x.x based compiler . clang 3.x IS VERY OLD AND COMPLETELY OUTDATED! 
also downloading a clang 6.x by your unrealengine script is completely ridicoulous, please make your scripts use already LOCAL installed RECENT MODERN compilers!