The error message you will see when building may look something like:

clang -pthread -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -fprofile-instr-generate -flto Parser/acceler.o Parser/grammar1.o Parser/listnode.o Parser/node.o Parser/parser.o Parser/bitset.o Parser/metagrammar.o Parser/firstsets.o Parser/grammar.o Parser/pgen.o Objects/obmalloc.o Python/dynamic_annotations.o Python/mysnprintf.o Python/pyctype.o Parser/tokenizer_pgen.o Parser/printgrammar.o Parser/parsetok_pgen.o Parser/pgenmain.o -lpthread -ldl  -lutil -o Parser/pgen
/usr/bin/ld: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so: error loading plugin: /usr/lib/llvm-3.8/bin/../lib/LLVMgold.so: cannot open shared object file: No such file or directory
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Makefile:793: recipe for target 'Parser/pgen' failed
make[2]: *** [Parser/pgen] Error 1

as seen on ubuntu 16.04.

Related to https://bugs.launchpad.net/ubuntu/+source/llvm-toolchain-snapshot/+bug/1254970 which suggests we also need -B/usr/lib/gold-ld in our LDFLAGS and/or CFLAGS when using clang on Linux with LTO.

testing with it set in both still runs into errors; someone with clang LTO experience will need to untangle this.

(is this related to why --with-lto doesn't work on MacOS as well?)

Looks like a package dependency issue: installing ``llvm-dev`` package should fix the problem. Or in that particular case ``llvm-3.8-dev``.

I confirm that it's currently broken. Test on Fedora 28, clang version 6.0.1 (tags/RELEASE_601/final):

$ ./configure --with-pydebug CC=clang --with-lto  && make
(...)
checking for x64 gcc inline assembler... yes
checking whether float word ordering is bigendian... unknown
configure: error: 

Unknown float word ordering. You need to manually preset
ax_cv_c_float_words_bigendian=no (or yes) according to your system.

New changeset 5ad36f9b21a3aa3b2265b1b43d73522cc3322df2 by Victor Stinner (serge-sans-paille) in branch 'master':
bpo-28015: Support LTO build with clang (GH-9908)
https://github.com/python/cpython/commit/5ad36f9b21a3aa3b2265b1b43d73522cc3322df2

New changeset 69a3f153a92fd8c86080e8da477ee50df18fc0d1 by Miss Islington (bot) in branch '3.7':
bpo-28015: Support LTO build with clang (GH-9908)
https://github.com/python/cpython/commit/69a3f153a92fd8c86080e8da477ee50df18fc0d1

The issue has been fixed in Python 3.7 and master.

If someone is volunteer to backport it to older version, please go ahead :-) But according to serge-sans-paille, it's non trivial and I'm not sure that it's worth it. Sorry, usually we focus on the master branch for "new features".
https://github.com/python/cpython/pull/9908#issuecomment-433896161

So I close the issue.

Thanks serge-sans-paille for this nice enhancement!

The issue with the 3.6 branch is https://bugs.python.org/issue31625 which hasn't been backported to 3.6. Would it make sense to backport it?

I have the backports ready locally but not sure if pushing https://bugs.python.org/issue31625 to 3.6 is desirable.

Probably that's fine, but can #31625 be easily worked around by, e.g., using make AR=llvm-ar RANLIB=llvm-ranlib?

The workaround indeed works.

New changeset f83ee476d48dbeb90ddf3526b04936a49a87973a by Ned Deily (stratakis) in branch '3.6':
bpo-28015: Support LTO build with clang (GH-9908) (GH-10922)
https://github.com/python/cpython/commit/f83ee476d48dbeb90ddf3526b04936a49a87973a

See also Issue31354, Issue35351, and Issue35257.