On Fedora 29 x64 with clang 7 installed. clang --version gives:
clang version 7.0.1 (Fedora 7.0.1-6.fc29)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Tried to start mach configure on mozilla-central and it kept erroring:
0:06.74 checking for llvm-config... /usr/bin/llvm-config
0:06.80 ERROR: Could not find the clang shared library in the path /usr/lib64
0:06.80 returned by llvm-config --libdir (searched for files [u'libclang.so.1', u'libclang.so']).

Checking /usr/lib64 shows there is a libclang.so.7 but no libclang.so link to libclang.so.7

Setting .mozconfig with:
export CC=/.mozbuild/clang/bin/clang
export CXX=/.mozbuild/clang/bin/clang++

didn't help, but leaving .mozconfig with:
export CC=clang
export CXX=clang++

and adding $HOME/.mozbuild/clang/bin to the PATH did fix it.

Expected result:

1. mach configure would work with system installed clang 7, should look for libclang.so.<major version number> too