In case pkgconfig is not available on the system, an alternate helper script is provided: libupsclient-config.
It will be installed in the same directory as NUT client programs (BINDIR), providing that you have enabled the --with-dev flag to the configure script.
The usage is about the same as pkg-config and similar tools.
To get CFLAGS, use:
$ libupsclient-config --cflags
To get LD_FLAGS, use:
$ libupsclient-config --libs
References: libupsclient-config(1) manual page,
this feature may evolve (name change), or even disappear in the future.