I'm trying to build JPostal but I get the following error after issuing the command ./gradlew assemble:

checking for gcj... no
checking for guavac... no
checking for jikes... no
checking for javac... javac
checking if javac works... yes
checking for C:\x\Java\jdk1.8.0_191/include/jni.h... yes
checking for stdlib.h... (cached) yes
checking for string.h... (cached) yes
checking for size_t... yes
checking for uint16_t... yes
checking for GNU libc compatible malloc... yes
checking for strdup... yes
checking for pkg-config... /usr/bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
checking for LIBPOSTAL... yes
checking that generated files are newer than configure... done
configure: creating ./config.status
config.status: creating Makefile
config.status: creating src/main/c/Makefile
config.status: creating config.h
config.status: executing depfiles commands
config.status: executing libtool commands

Making install in src/main/c
make[1]: Entering directory '/c/x/ProgramFiles/JPostal/src/main/c'
/bin/sh ../../../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I../../..     -g -O2 -IC:\x\Java\jdk1.8.0_191/include -MT libjpostal_parser_la-jpostal_AddressParser.lo -MD -MP -MF .deps/libjpostal_parser_la-jpostal_AddressParser.Tpo -c -o libjpostal_parser_la-jpostal_AddressParser.lo `test -f 'jpostal_AddressParser.c' || echo './'`jpostal_AddressParser.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I../../.. -g -O2 -IC:xJavajdk1.8.0_191/include -MT libjpostal_parser_la-jpostal_AddressParser.lo -MD -MP -MF .deps/libjpostal_parser_la-jpostal_AddressParser.Tpo -c jpostal_AddressParser.c  -DDLL_EXPORT -DPIC -o .libs/libjpostal_parser_la-jpostal_AddressParser.o
jpostal_AddressParser.c:1:10: fatal error: jni.h: No such file or directory
    1 | #include <jni.h>
      |          ^~~~~~~
compilation terminated.
make[1]: *** [Makefile:473: libjpostal_parser_la-jpostal_AddressParser.lo] Error 1
make[1]: Leaving directory '/c/x/ProgramFiles/JPostal/src/main/c'
make: *** [Makefile:404: install-recursive] Error 1
:buildJniLib FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':buildJniLib'.
> Process 'command 'sh'' finished with non-zero exit value 2

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 58.914 secs

I installed LibPostal exactly as described here so why are the file jni.h, gcj, guavac and jikes libs missing?

I looked at GitHub and I didn't see any open issues addressing this question.

UPDATE 1:

In the screenshot below you can see that the jni.h' file is clearly present in /include/` folder: