FreeBSD build error: 'flatbuffers/flatbuffers.h' file not found

[MartialM1nd]

Build fails with error on a FreeBSD 14.2-RELEASE thin jail.

Followed FreeBSD install directions from README.md

I tried adding /usr/local/include to my environment path and "-I/usr/local/include" flag to the gmake command.
Same error.

gmake -I/usr/local/include

c++ -std=c++20 -O3 -g -Wall -fPIC -DDOCOPT_HEADER_ONLY -Iinclude -Ibuild -Isrc -Igolpe/external -Igolpe/external/lmdbxx/include -Igolpe/external/config/include -Igolpe/external/json/include -Igolpe/external/PEGTL/include -Igolpe/external/hoytech-cpp -Igolpe/external/docopt.cpp -Igolpe/external/loguru -Igolpe/external/parallel-hashmap -Iexternal/negentropy/cpp -MMD -MP -MT golpe/logging.o -MF golpe/logging.d -c golpe/logging.cpp -o golpe/logging.o
In file included from golpe/logging.cpp:1:
build/golpe.h:16:10: fatal error: 'flatbuffers/flatbuffers.h' file not found
16 | #include "flatbuffers/flatbuffers.h"
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
gmake: *** [golpe/rules.mk:32: golpe/logging.o] Error 1

ls -l /usr/local/include/flatbuffers/

total 260
-rw-r--r-- 1 root wheel 2588 Jan 24 18:36 allocator.h
-rw-r--r-- 1 root wheel 9201 Jan 24 18:36 array.h
-rw-r--r-- 1 root wheel 17446 Jan 24 18:36 base.h
-rw-r--r-- 1 root wheel 7376 Jan 24 18:36 buffer.h
-rw-r--r-- 1 root wheel 1511 Jan 24 18:36 buffer_ref.h
-rw-r--r-- 1 root wheel 2978 Jan 24 18:36 code_generator.h
-rw-r--r-- 1 root wheel 8288 Jan 24 18:36 code_generators.h
-rw-r--r-- 1 root wheel 2381 Jan 24 18:36 default_allocator.h
-rw-r--r-- 1 root wheel 3256 Jan 24 18:36 detached_buffer.h
-rw-r--r-- 1 root wheel 1399 Jan 24 18:36 file_manager.h
-rw-r--r-- 1 root wheel 62731 Jan 24 18:36 flatbuffer_builder.h
-rw-r--r-- 1 root wheel 10998 Jan 24 18:36 flatbuffers.h
-rw-r--r-- 1 root wheel 4038 Jan 24 18:36 flatc.h
-rw-r--r-- 1 root wheel 1220 Jan 24 18:36 flex_flat_util.h
-rw-r--r-- 1 root wheel 66984 Jan 24 18:36 flexbuffers.h
-rw-r--r-- 1 root wheel 9932 Jan 24 18:36 grpc.h
-rw-r--r-- 1 root wheel 3627 Jan 24 18:36 hash.h
-rw-r--r-- 1 root wheel 47300 Jan 24 18:36 idl.h
-rw-r--r-- 1 root wheel 13657 Jan 24 18:36 minireflect.h
drwxr-xr-x 2 root wheel 4 May 23 09:40 pch
-rw-r--r-- 1 root wheel 22086 Jan 24 18:36 reflection.h
-rw-r--r-- 1 root wheel 58569 Jan 24 18:36 reflection_generated.h
-rw-r--r-- 1 root wheel 4520 Jan 24 18:36 registry.h
-rw-r--r-- 1 root wheel 19107 Jan 24 18:36 stl_emulation.h
-rw-r--r-- 1 root wheel 2198 Jan 24 18:36 string.h
-rw-r--r-- 1 root wheel 1577 Jan 24 18:36 struct.h
-rw-r--r-- 1 root wheel 6756 Jan 24 18:36 table.h
-rw-r--r-- 1 root wheel 26593 Jan 24 18:36 util.h
-rw-r--r-- 1 root wheel 13491 Jan 24 18:36 vector.h
-rw-r--r-- 1 root wheel 8666 Jan 24 18:36 vector_downward.h
-rw-r--r-- 1 root wheel 13494 Jan 24 18:36 verifier.h

Any ideas?
Happy to provide more info.