openSUSE 15.3 liibmnl-revel package puts the llibmnl header file ibmnl.h in /usr/include/libmnl/libmnl . I suspect the reason for the double sub-directory is because in previous incarnations, the upper sub-directory was versioned eg /usr/include/libnml-0.1

The issue #223 addressed the generic case of libmnl.h detection with the appropriate solution of using pkg-config to find the header file eg pkg-config --cflags libmnl but for unknown reasons that issue was closed without any comment or resolution.

The configure option "--with-libmnl=path specify explicit path for libmnl" to specify a path does not actually not do anything in respect of a path. The variable is only used to test if it has been set, not whether it has any value. One would expect ta variable to be something like --with-libmnl-includes and --with-libmlnl-libdir to specify paths to override pkg-config path searches.

This issue is present in the newest release 0.15.0, and on checking the very latest Github version of configure.ac, there is no change to the hard coded path searching with the use of pkg-config. In fact the configure.ac comments say ipkg-config is only used for the options enable_systemd, with_pyunbound, and with_pythonmod", and not to require it otherwise.