Fix installation of cmake config when building with cmake (#1161)

When building with the auto tools, the manually mantained
harfbuzz-config.cmake is installed. When building with cmake, we can use
cmake to generate the correct config files for us and install them.

CMakeLists.txt

@@ -761,11 +761,16 @@ endif ()
 
 if (NOT SKIP_INSTALL_LIBRARIES AND NOT SKIP_INSTALL_ALL)
   install(TARGETS harfbuzz
+    EXPORT harfbuzzConfig
     ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
     FRAMEWORK DESTINATION Library/Frameworks
   )
+  install(EXPORT harfbuzzConfig
+      NAMESPACE harfbuzz::
+      DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/harfbuzz
+  )
   if (HB_BUILD_UTILS)
     install(TARGETS hb-view
       RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}