Install cmake package and pkgconfig files. Closes #954927
As the title suggests install cmake package files and also .pc file.
- .pc file seems to me platform independent so I chose /usr/share over /usr/lib/${deb_platform_smth}/
 - Behavior changed to upstream by introducing 
Armadillo::namespace. I chose that approach becauseensmallenlibrary usesArmadillo::Armadilloas imported library. (But I might miss any big picture here and don't know how other dependent libraries select that target.) 
cmake-data includes FindArmadillo.cmake, therefore to use this cmake package, one should use the CONFIG switch
find_package(Armadillo CONFIG)
...
target_link_libraries(mytarget PRIVATE Armadillo::Armadillo)
Also the namespace part is probably best included upstream...