Compile error while building postgresql 10.3

I'm new to PostgreSQL but not to Unix or databases, and am reading the
(wonderful) docs and learning. I tried to determine which mailing list to
send this to, and this one seems to fit best.

Just for fun, I am trying to build postgres from source on FreeBSD 11. Yes,
I know I don't need to, and I have already installed the 10.3 server and
client packages, and they run fine. I did a 'git clone' today, and have
hours-old source code.

So, I install the prereqs, and did a 'configure' and 'make', and expected
it to 'just work'. However, I get this compile error:

cc -I/usr/local/include -I../../src/port -DFRONTEND -I../../src/include
-c -o pg_crc32c_sse42.o pg_crc32c_sse42.c
pg_crc32c_sse42.c:37:18: error: always_inline function '_mm_crc32_u64'
requires
target feature 'ssse3', but would be inlined into function
'pg_comp_crc32c_sse42' that is compiled without support for 'ssse3'
crc = (uint32) _mm_crc32_u64(crc, *((const uint64 *) p));
^
pg_crc32c_sse42.c:44:9: error: always_inline function '_mm_crc32_u32'
requires
target feature 'ssse3', but would be inlined into function
'pg_comp_crc32c_sse42' that is compiled without support for 'ssse3'
crc = _mm_crc32_u32(crc, *((const unsigned int *) p));
^
pg_crc32c_sse42.c:63:9: error: always_inline function '_mm_crc32_u8'
requires
target feature 'ssse3', but would be inlined into function
'pg_comp_crc32c_sse42' that is compiled without support for 'ssse3'
crc = _mm_crc32_u8(crc, *p);
^
3 errors generated.

I googled, and search the archives, and don't see anything applicable. My C
compiler is:

$ cc --version
FreeBSD clang version 4.0.0 (tags/RELEASE_400/final 297347) (based on LLVM
4.0.0)
Target: x86_64-unknown-freebsd11.1
Thread model: posix

I don't know what else you might want to know, but I'll supply it, if you
ask.

Any ideas what's wrong?