g++ %{optflags} -I%{_jnidir}/include -I%{_jnidir}/include/linux -fPIC -shared
-o libjSSC-%{LIBRARY_VERSION}.so src/cpp/_nix_based/jssc.cpp

expands to 

g++ -O2 -g -m64 -fmessage-length=0 -D_FORTIFY_SOURCE=2 -fstack-protector
-funwind-tables -fasynchronous-unwind-tables -I/usr/lib64/java/include
-I/usr/lib64/java/include/linux -fPIC -shared -o libjSSC-.so
src/cpp/_nix_based/jssc.cpp
src/cpp/_nix_based/jssc.cpp:46:17: fatal error: jni.h: No such file or
directory

ls -a /usr/lib64/java/
.  ..


ls /usr/lib64/jvm/java/include/
classfile_constants.h  jawt.h  jdwpTransport.h  jni.h  jvmticmlr.h  jvmti.h 
linux

no macro documented here https://en.opensuse.org/openSUSE:Java_RPM_Macros
provides that location.