Use cmake 3.1+ compile features

fixes issue #9

2.0.2

fix issue8: refactor attached target dependencies

1.0.2 - Fix for issue #6

• fix for issue #6

1.0.1 General revamp

This release provides the following:

• rename function attach_llvmir_target() to attach_llvmir_bc_target()
  (the former is still defined but now deprecated)
• proper definition of attached target properties via CMake's define_property() function
• tested with lower CMake version (2.8.11) in order to determine compatibility
• improved error checking
  • failure upon error (stop further processing)
  • unified and improved checks on expected target attributes

1.0.0

update readme