When building NetBSD, you can set
to also build clang and install it, and additionally
if you want to use it to compile NetBSD itself during build.sh.
When you have a NetBSD compiled this way, you can set
PKGSRC_COMPILER= clang CLANGBASE= /usr HAVE_LLVM= yes
in your /etc/mk.conf to use it. You must do that before building packages, especially libtool. It's usually fine to link binaries generated from gcc to those generated with clang or vice versa.