DIY include directories not listed in makefile export

The makefile export (vtkm_config.mk) and the package export (vtkm.pc) have compiler flags to the include directory. However, they do not have links to the third-party libraries, which are in the vtkm/thirdparty directory, each with their own subdirectory. These should either also be listed as include directories, installed instead to the main include directory, or have a symbolic link installed to the appropriate place.