Debug build of WebKit fails on Linux with Ruby 2.0

The following was spotted in our weekly CI debug build on Oracle Linux 7.x:

$ gradle -PCONF=DebugNative -PCOMPILE_WEBKIT=true ... all
...
Scanning dependencies of target LowLevelInterpreterLib
[ 26%] Building CXX object Source/JavaScriptCore/CMakeFiles/LowLevelInterpreterLib.dir/llint/LowLevelInterpreter.cpp.o
.../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:89:in `parse': undefined method `/' for #<Pathname:working_directory> (NoMethodError)
from .../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:129:in `test'
from .../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:208:in `block in selftest'
from .../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:207:in `each'
from .../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:207:in `selftest'
from .../jfx/modules/javafx.web/build/linux/Debug/DerivedSources/ForwardingHeaders/JavaScriptCore/Scripts/resolve-asm-file-conflicts.rb:398:in `<main>'
Error running cmd: pid 14478 exit 1
gmake[2]: *** [Source/JavaScriptCore/CMakeFiles/LowLevelInterpreterLib.dir/llint/LowLevelInterpreter.cpp.o] Error 1
gmake[1]: *** [Source/JavaScriptCore/CMakeFiles/LowLevelInterpreterLib.dir/all] Error 2
gmake: *** [all] Error 2

> Task :web:compileNativeLinux FAILED

I wasn't able to reproduce it locally on Ubuntu (16.04). It might be related to the version of ruby or some other tool on our build machine.