llvm: don't link against libxml2

On Big Sur, when compiling LLVM 10, you end up with a
garbage `-llibxml2.tdd` in the default system libs. It
doesn't seem to be needed.

Author: maxfierke

omnibus/config/software/llvm.rb

@@ -45,6 +45,7 @@
     " -DLLVM_ENABLE_ASSERTIONS=ON" \
     " -DLLVM_INCLUDE_TESTS=OFF" \
     " -DLLVM_ENABLE_Z3_SOLVER=OFF" \
+    " -DLLVM_ENABLE_LIBXML2=OFF" \
     "#{' -DPYTHON_EXECUTABLE=$(which python2.7)' if centos? }"\
     " #{project_dir}", env: env, cwd: llvm_build_dir
   command "cmake --build .", env: env, cwd: llvm_build_dir