HI Máté,
Make sure you build both ‘clang’ and ‘clang-headers’, the latter actually copies the headers to the build directory.
The install should be relocatable, but make sure to also include the internal headers and put them into the following structure:
/bin/clang
/bin/clang++
/bin/… ← other binaries/symlinks go here
/lib//include/ ← headers should go here