Create macro to facilitate find_package and setup libraries

[egonzalf]

Create a macro or maybe a function in cmake that will concentrate all the statements currently needed to setup a depency of an external library, like BLAS.

Currently the user needs to invoke find_package and then deal with the FOUND/NOT_FOUND conditions. Also, the user needs to set the correct variables for the include paths, library links, etc.

The idea is to try to facilitate all of this, if possible.