CMake is not required to build gem5. It is only required to build
and link the optional DRAMSysm library. Therefore, if the DRAMSys repo
has been cloned but CMake is not present this patch ensures no attempt
at building or linking DRAMSysm is made. A warning is thrown
inform the user of the missing CMake.
Change-Id: I4d22e3a16655fd90f6b109b4e75859628f7d532d