cc -c -fPIC -I/usr/local/include/luajit-2.1 -DNGX_CLOJURE_BE_SILENT_WITHOUT_JVM -I/usr/local/openjdk8/include -I/usr/local/openjdk8/include/freebsd -O2 -pipe  -g -I/usr/local/include -DLIBICONV_PLUG -fstack-protector-strong -DLDAP_DEPRECATED -isystem /usr/local/include -fno-strict-aliasing  -DLARGEFILE_SOURCE -DBUILDING_NGINX -DNDK_SET_VAR -Wno-deprecated-declarations -DNDK_SET_VAR -DNDK_SET_VAR -DMONGO_HAVE_STDINT -Isrc -Wno-error -D_MONGO_USE_GETADDRINFO -DNDK_SET_VAR -DNDK_SET_VAR -DNDK_SET_VAR -DNDK_SET_VAR -DNDK_UPSTREAM_LIST -I/usr/local/include/ImageMagick-6 -DMAGICKCORE_HDRI_ENABLE=0 -DMAGICKCORE_QUANTUM_DEPTH=16 -I src/core  -I src/event  -I src/event/modules  -I src/os/unix  -I src/http/modules/perl  -I /usr/ports/www/nginx/work/nginx_upstream_check_module-9aecf15  -I /usr/ports/www/nginx/work/ngx_devel_kit-0.3.1/objs  -I objs/addon/ndk  -I /usr/ports/www/nginx/work/ngx_devel_kit-0.3.1/src  -I /usr/ports/www/nginx/work/ngx_devel_kit-0.3.1/objs  -I objs/addon/ndk  -I /usr/local/include  -I /usr/local/include  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ddebug.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_module.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_handler.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_processor.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_upstream.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_util.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_output.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/resty_dbd_stream.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_keepalive.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_quoting.h  -I /usr/ports/www/nginx/work/drizzle-nginx-module-0.1.11/src/ngx_http_drizzle_checker.h  -I src/stream  -I /usr/ports/www/nginx/work/ngx_dynamic_upstream-960eef2/src  -I %%GSSAPINCDIR%%  -I /usr/ports/www/nginx/work/nchan-1.2.7/src  -I /usr/ports/www/nginx/work/nginx-push-stream-module-0.5.4/src  -I /usr/ports/www/nginx/work/nginx-push-stream-module-0.5.4/include  -I /usr/ports/www/nginx/work/nginx_upstream_module-1278ee5/src  -I /usr/ports/www/nginx/work/nginx_upstream_module-1278ee5/third_party  -I /usr/local/include/msgpuck  -I /usr/local/include/yajl  -I /usr/ports/www/nginx/work/nginx-video-thumbextractor-module-f5b5bae/include  -I /usr/ports/www/nginx/work/nginx-video-thumbextractor-module-f5b5bae/src  -I /usr/local/include/luajit-2.1  -I /usr/ports/www/nginx/work/lua-nginx-module-0.10.19/src/api  -I /usr/ports/www/nginx/work/njs-0.5.2/nginx/../src  -I /usr/ports/www/nginx/work/njs-0.5.2/nginx/../build  -I /usr/ports/www/nginx/work/njs-0.5.2/nginx/../src  -I /usr/ports/www/nginx/work/njs-0.5.2/nginx/../build  -I /usr/local/include  -I /usr/ports/www/nginx/work/nginx-rtmp-module-791b613  -I /usr/local/include/libxml2  -I /usr/local/include  -I /usr/ports/www/nginx/work/passenger-6.0.8/src  -I /usr/local/include/libxml2  -I /usr/local/include  -I objs  -I src/http  -I src/http/modules  -I src/http/v2  -I /usr/ports/www/nginx/work/nginx_ajp_module-a964a0b  -I /usr/ports/www/nginx/work/ngx_devel_kit-0.3.1/src  -I /usr/ports/www/nginx/work/nginx-clojure-0.5.2/src/c  -I src/mail  -I src/stream  -o objs/addon/c/ngx_http_clojure_mem.o  /usr/ports/www/nginx/work/nginx-clojure-0.5.2/src/c/ngx_http_clojure_mem.c
In file included from /usr/ports/www/nginx/work/nginx-clojure-0.5.2/src/c/ngx_http_clojure_mem.c:6:
/usr/ports/www/nginx/work/nginx-clojure-0.5.2/src/c/ngx_http_clojure_jvm.h:8:10: fatal error: 'jni.h' file not found
#include <jni.h>
         ^~~~~~~
1 error generated.
*** Error code 1

Stop.
make[3]: stopped in /usr/ports/www/nginx/work/nginx-1.20.0
*** Error code 1

Stop.
make[2]: stopped in /usr/ports/www/nginx/work/nginx-1.20.0
*** Error code 1

Stop.
make[1]: stopped in /usr/ports/www/nginx
*** Error code 1

Stop.
make: stopped in /usr/ports/www/nginx

===>>> make build failed for www/nginx
===>>> Aborting update

===>>> Upgrade of expat-2.2.10 to expat-2.3.0 complete


===>>> You can restart from the point of failure with this command line:
       portmaster <flags> www/nginx www/lua-resty-core www/lua-resty-lrucache

This command has been saved to ~/portmasterfail.txt

================================================================================

It appears to have to do with the Clojure add-on to nginx. I can live without it right now as I enabled every option for novelty's sake and barely even know how to use Clojure. locate confirms the presence of several jni.h files on the system. In my case these are:

/usr/local/go/misc/cgo/test/testdata/issue26213/jni.h
/usr/local/include/libavcodec/jni.h
/usr/local/openjdk16/include/jni.h
/usr/local/openjdk7/include/jni.h
/usr/ports/lang/gcc10/work/gcc-10.2.0/libgo/misc/cgo/test/testdata/issue26213/jni.h
/usr/ports/multimedia/ffmpeg/work/ffmpeg-4.4/libavcodec/ffjni.h
/usr/ports/multimedia/ffmpeg/work/ffmpeg-4.4/libavcodec/jni.h
/usr/ports/multimedia/ffmpeg/work/stage/usr/local/include/libavcodec/jni.h

I would offer more if I could but alas I haven't even been using FreeBSD for an entire week at the time of writing!