Improve out-of-source build

Hello,

I improved the out-of-source build for your library, and it might prove to be useful to you too.
Here is what I did:
 - renamed the CMake variable "PREFIX" into "EXTERNAL_PREFIX" in order to reuse it multiples times throughout the configuration, while keeping its meaning
 - Modified the EXTERNAL_PREFIX variable to refer to the build directory
 - Fixed a few paths by using the EXTERNAL_PREFIX variable

I hope you'll make use of it !