When built with Autotools, dbus installs a .pc file for libdbus into ${libdir}/pkg-config. It should ideally do the same in CMake builds, although if Windows people have never needed this functionality, that's far from being a high priority.

The code would look something like this (part of a patch by Руслан Ижбулатов on Bug #75858):

configure_file(${CMAKE_SOURCE_DIR}/../dbus-1.pc.in ${CMAKE_BINARY_DIR}/../dbus-1.pc @ONLY)
install(FILES ${CMAKE_BINARY_DIR}/../dbus-1.pc DESTINATION ${EXPANDED_LIBDIR}/pkgconfig)

Expanding all the variables in the .pc correctly would additionally require something like this:

# Following Autotools naming, for substitution into *.pc, since we share
# Autotools' *.pc.in files.
set(bindir                   ${exec_prefix}/bin)
set(datarootdir              ${prefix}/${DATAROOTDIR})
set(datadir                  ${prefix}/${DATADIR})
set(sysconfdir               ${prefix}/etc)
set(dbus_daemondir           ${bindir})

I have not tested the above, but maybe it already works.

For bonus points you could let pkg-config perform the variable expansion so that the .pc file is relocatable, like it is on Autotools after fixing Bug #75858.