cmake: Check for _GNU_SOURCE and add it if available.
cmake: Check for _GNU_SOURCE and add it if available.

This is needed for strsignal().

jschmidlapp/cmocka,aleksandr-vin/cmock,JonathonReinhart/cmocka,nuumio/cmocka,chadversary/cmocka,Odie/cmocka,dsch/cmocka,jhrozek/cmocka,clibs/cmocka,chadversary/cmocka,Odie/cmocka,akikoskinen/cmocka,tyc/cmocka,jamesmunns/CMocka,nuumio/cmocka,nuumio/cmocka,lsjostro/cmocka,linuxmaniac/cmocka,wingyplus/cmocka,dsch/cmocka,JonathonReinhart/cmocka,jschmidlapp/cmocka,jhrozek/cmocka,jamesmunns/CMocka,wingyplus/cmocka,jschmidlapp/cmocka,Odie/cmocka,VladimirTyrin/cmocka,linuxmaniac/cmocka,lsjostro/cmocka,tyc/cmocka,aleksandr-vin/cmock,VladimirTyrin/cmocka,tyc/cmocka,jhrozek/cmocka,clibs/cmocka