Change CMAKE version requirement to 2.8
Change CMAKE version requirement to 2.8

ExternalProject is a feature added in version 2.8

linan7788626/clBLAS,guacamoleo/clBLAS,hemmingway/clBLAS,guacamoleo/clBLAS,ghisvail/clBLAS,hemmingway/clBLAS,devfreesg/clBLAS,arrayfire/clBLAS,hughperkins/clBLAS,clMathLibraries/clBLAS,clMathLibraries/clBLAS,clMathLibraries/clBLAS,lunochod/clBLAS,devfreesg/clBLAS,linan7788626/clBLAS,guacamoleo/clBLAS,tingxingdong/clean,BenjaminCoquelle/clBLAS,TimmyLiu/clBLAS,tmagomedov/clBLAS,TimmyLiu/clBLAS,TimmyLiu/clBLAS,guacamoleo/clBLAS,linan7788626/clBLAS,tmagomedov/clBLAS,hughperkins/clBLAS,tingxingdong/clean,BenjaminCoquelle/clBLAS,kknox/clBLAS,devfreesg/clBLAS,arrayfire/clBLAS,TimmyLiu/clBLAS,linan7788626/clBLAS,devfreesg/clBLAS,tingxingdong/clean,BenjaminCoquelle/clBLAS,hemmingway/clBLAS,tmagomedov/clBLAS,ghisvail/clBLAS,hemmingway/clBLAS,hughperkins/clBLAS,kknox/clBLAS,kknox/clBLAS,arrayfire/clBLAS,lunochod/clBLAS,BenjaminCoquelle/clBLAS,arrayfire/clBLAS,lunochod/clBLAS,tmagomedov/clBLAS,tingxingdong/clean,lunochod/clBLAS,ghisvail/clBLAS,ghisvail/clBLAS,clMathLibraries/clBLAS,kknox/clBLAS,hughperkins/clBLAS