I maintain the FreeBSD packages for LLVM. At a user's request I added
flang support to the build of our llvm-devel port (this tracks the LLVM
master branch). Unfortunately, I discovered that the build failed when
compiled with LLVM_LINK_LLVM_DYLIB=ON so I've not currently enabled it
because the resulting package is too large without that option (over
500MB as a .txz).

It would be helpful if LLVM_BUILD_LLVM_DYLIB=ON worked as is greatly
reduces binary size.

The failure I get is:

FAILED: tools/flang/include/flang/__fortran_builtins.mod
cd /usr/local/poudriere/ports/brooks/devel/llvm-devel/work/.build/tools/flang/include/flang && /usr/local/poudriere/ports/brooks/devel/llvm-devel/work/.build/bin/f18 -fparse-only -I/usr/local/poudriere/ports/brooks/devel/llvm-devel/work/.build/tools/flang/include/flang /usr/local/poudriere/ports/brooks/devel/llvm-devel/work/llvm-project-30c1633386e7cfb01c0a54b31ccf4c3a3873e71b/flang/module/__fortran_builtins.f90
: CommandLine Error: Option 'help-list' registered more than once!
LLVM ERROR: inconsistency in registered CommandLine options

I'm happy to file a bugreport if that would be useful.