CMake fails for old versions of cmake

[Yina Geng]

In CMake/freud/FREUDCFlagsSetup.cmake, VERSION_GREATER_EQUAL does not exist in older versions of CMake. Changing to VERSION_GREATER works (although the logic is now different).

[Bradley Dice]

This comparison operation was introduced in cmake 3.7.

@harperic corrected the bug on 2017-08-21 with commits:

• a2ef6c74fbf96ef5b7cb3e102a6301fca5a0f59f
• 272a71ec99de364f8924ae3a54fd5ed4d59dacc1