The issue appears to be fixed in the siurces.

Assuming you are attempting to build XEmacs 21.5 (although this is applicable for 21.4), pkgsrc has a patch at <http://cvsweb.netbsd.org/bsdweb.cgi/pkgsrc/editors/xemacs-current/patches/patch-src_lisp.h?rev=1.2&content-type=text/x-cvsweb-markup>

uname -a
Darwin Georges-iMac.local 18.7.0 Darwin Kernel Version 18.7.0: Tue Nov 10 
00:07:31 PST 2020; root:xnu-4903.278.51~1/RELEASE_X86_64 x86_64

when I run make I am getting the following error

gcc -c -g -O3 -Wall -Wno-switch -Winline -Wmissing-prototypes -Wsign-
compare -Wshadow -Wpointer-arith  -Demacs -I. -DHAVE_CONFIG_H abbrev.c
In file included from abbrev.c:35:
./lisp.h:204:3: error: typedef redefinition with different types ('union 
max_align_t' vs
      'long double')
} max_align_t;
  ^
/Library/Developer/CommandLineTools/usr/lib/clang/10.0.1/include/__stddef_
max_align_t.h:32:21: note: 
      previous definition is here
typedef long double max_align_t;
                    ^
1 error generated.
make[1]: *** [abbrev.o] Error 1
make: *** [src] Error 2