Adds missing DEFINED in check for 64 bit system in CMakeLists.txt

[Jochen0x90h]

I want to install into lib on a 64 bit system. For this I pass -DLIB_SUFFIX="" to CMake. This didn't work because NOT LIB_SUFFIX evaluates to false if LIB_SUFFIX is defined but empty string.

[spaghetti-]

Good catch, I added in some checks of my own for MacOS as well as changed the default install location. I combined your commit in #57 I hope you don't mind.