Fix the Cmake GCC identifier
Fix the Cmake GCC identifier

GCC is 'GNU', not 'GCC'.

marono/cli,MichaelSimons/cli,Faizan2304/cli,jkotas/cli,schellap/cli,stuartleeks/dotnet-cli,krwq/cli,harshjain2/cli,naamunds/cli,jaredpar/cli,blackdwarf/cli,gkhanna79/cli,FubarDevelopment/cli,piotrpMSFT/cli-old,Faizan2304/cli,gkhanna79/cli,svick/cli,mylibero/cli,danquirk/cli,jaredpar/cli,ravimeda/cli,jonsequitur/cli,EdwardBlair/cli,krwq/cli,danquirk/cli,livarcocc/cli-1,borgdylan/dotnet-cli,blackdwarf/cli,jkotas/cli,krwq/cli,blackdwarf/cli,piotrpMSFT/cli-old,stuartleeks/dotnet-cli,JohnChen0/cli,jonsequitur/cli,danquirk/cli,borgdylan/dotnet-cli,gkhanna79/cli,dasMulli/cli,mylibero/cli,ravimeda/cli,johnbeisner/cli,livarcocc/cli-1,marono/cli,nguerrera/cli,schellap/cli,JohnChen0/cli,nguerrera/cli,svick/cli,nguerrera/cli,johnbeisner/cli,harshjain2/cli,weshaggard/cli,jkotas/cli,schellap/cli,schellap/cli,mlorbetske/cli,mlorbetske/cli,johnbeisner/cli,marono/cli,naamunds/cli,mlorbetske/cli,marono/cli,AbhitejJohn/cli,FubarDevelopment/cli,borgdylan/dotnet-cli,FubarDevelopment/cli,stuartleeks/dotnet-cli,harshjain2/cli,AbhitejJohn/cli,nguerrera/cli,schellap/cli,weshaggard/cli,MichaelSimons/cli,jaredpar/cli,jaredpar/cli,AbhitejJohn/cli,naamunds/cli,MichaelSimons/cli,krwq/cli,JohnChen0/cli,danquirk/cli,weshaggard/cli,mylibero/cli,stuartleeks/dotnet-cli,danquirk/cli,FubarDevelopment/cli,EdwardBlair/cli,MichaelSimons/cli,jaredpar/cli,jkotas/cli,MichaelSimons/cli,jonsequitur/cli,weshaggard/cli,schellap/cli,borgdylan/dotnet-cli,mylibero/cli,JohnChen0/cli,krwq/cli,piotrpMSFT/cli-old,jonsequitur/cli,mlorbetske/cli,ravimeda/cli,stuartleeks/dotnet-cli,jaredpar/cli,livarcocc/cli-1,AbhitejJohn/cli,dasMulli/cli,naamunds/cli,jkotas/cli,weshaggard/cli,anurse/Cli,piotrpMSFT/cli-old,blackdwarf/cli,Faizan2304/cli,dasMulli/cli,mylibero/cli,EdwardBlair/cli,piotrpMSFT/cli-old,piotrpMSFT/cli-old,gkhanna79/cli,naamunds/cli,marono/cli,gkhanna79/cli,svick/cli,borgdylan/dotnet-cli