Strange things when trying to build math/arb

I get the weirdest errors when I try to build the port math/arb, which 
was recently updated to version 2.21.0.  Maybe it is a compiler error?

Here is a sample:

/usr/bin/cc -DARB_BUILD_DLL -Darb_EXPORTS -I/usr/local/include 
-I/usr/ports/math/arb/work/arb-2.21.0 -O2 -pipe 
-fstack-protector-strong -fno-strict-aliasing -O2 -pipe 
-fstack-protector-strong -fno-strict-aliasing -fPIC -pthread -MD -MT 
CMakeFiles/arb.dir/acb_hypgeom/gamma_taylor.c.o -MF 
CMakeFiles/arb.dir/acb_hypgeom/gamma_taylor.c.o.d -o 
CMakeFiles/arb.dir/acb_hypgeom/gamma_taylor.c.o -c 
/usr/ports/math/arb/work/arb-2.21.0/acb_hypgeom/gamma_taylor.c
/usr/ports/math/arb/work/arb-2.21.0/acb_hypgeom/gamma_taylor.c:48:22:
/usr/ports/math/arb/work/arb-2.21.0/acb_hypgeom/gamma_taylor.c:87:35: 
error: use of undeclared identifier 'arb_hypgeom_gamma_coeffs'; did you 
mean 'arb_hypgeom_gamma_lower'?
         mag_mul_2exp_si(err, err, arb_hypgeom_gamma_coeffs[N].exp);
                                   ^~~~~~~~~~~~~~~~~~~~~~~~
It is as though the c compiler failed to preprocess the source code, and 
forget the include files.

Note, if I add USE_GCC=yes to the port Makefile, it builds just fine.

I am using:

cc --version
FreeBSD clang version 10.0.1 (git@github.com:llvm/llvm-project.git 
llvmorg-10.0.1-0-gef32c611aa2)

uname -a
FreeBSD wilberforce 12.3-PRERELEASE FreeBSD 12.3-PRERELEASE 
stable/12-n234170-35f1d655819 GENERIC  amd64