cmake 3.5 compatibility

[jagerman]

Adds a few minor workarounds to make CMakeLists.txt compatible with cmake 3.5 (which is still out in the wild as it is Ubuntu xenial's cmake version):

• $<IF isn't supported before 3.8, but all the uses of $<IF here are actually simpler to write without $<IF.

• VERSION_GREATER_EQUAL isn't supported before 3.7, but is easily worked around with a NOT ... VERSION_LESS

Fixes #547