cmake: Fix build with cmake-3.5.2.

[orbea]

For Slackware stable (14.2) it has the older cmake-3.5.2 which does not seem to support GREATER_EQUAL. So I replaced it with VERSION_EQUAL and VERSION_GREATER which is more verbose, but works with both newer and older cmake versions.

Unfortunately I am stuck testing against older cmake versions until Slackware 15.0 is out (When its ready).

CMake Error at cmake/SearchForStuff.cmake:225 (if):
  if given arguments:

    "GCC_VERSION" "GREATER_EQUAL" "9.0" "AND" "GCC_VERSION" "LESS" "9.2"

  Unknown arguments specified
Call Stack (most recent call first):
  CMakeLists.txt:27 (include)


-- Configuring incomplete, errors occurred!
See also "/tmp/SBo/pcsx2/build/CMakeFiles/CMakeOutput.log".
See also "/tmp/SBo/pcsx2/build/CMakeFiles/CMakeError.log".

[arcum42]

Works fine on my side with a newer version of cmake, tested both with gcc 9.1.0 and clang. I could go either way on which version of the check to use, but I'm fine with the current one, so I'll merge that.