I am also getting this from a clean git clone.  I am using MacOS (Mojave 10.14.6) and I just updated xcode to the latest version.
I did a fresh git clone of pytorch repostiory and started following the README directions. This is what I got:
Undefined symbols for architecture x86_64: "__cvtu32_mask16", referenced from: _xnn_f32_clamp_ukernel__avx512f in libXNNPACK.a(avx512f.c.o) _xnn_f32_dwconv_ukernel_up16x25__avx512f in libXNNPACK.a(up16x25-avx512f.c.o) _xnn_f32_dwconv_ukernel_up16x4__avx512f in libXNNPACK.a(up16x4-avx512f.c.o) _xnn_f32_dwconv_ukernel_up16x9__avx512f in libXNNPACK.a(up16x9-avx512f.c.o)

Here’s the exact order of what I did following the fresh git clone.
conda install numpy ninja pyyaml mkl mkl-include setuptools cmake cffi
	# Omitted `typing` because I’m on Python 3.7
git submodule sync
git submodule update --init —recursive
export CMAKE_PREFIX_PATH=${CONDA_PREFIX:-"$(dirname $(which conda))/../“}
USE_CUDA=0 MACOSX_DEPLOYMENT_TARGET=10.14 CC=clang CXX=clang++ python setup.py install

Clang
clang --version
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

GCC (same as clang)
gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/c++/4.2.1
Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin