Add policy CMP0035 to disallow variable_requires

Help/command/variable_requires.rst

Help/manual/cmake-policies.7.rst

Help/policy/CMP0035.rst

Source/cmPolicies.cxx

Source/cmPolicies.h

Source/cmVariableRequiresCommand.cxx

Source/cmVariableRequiresCommand.h

Tests/RunCMake/DisallowedCommands/CMP0035-NEW-result.txt

Tests/RunCMake/DisallowedCommands/CMP0035-NEW-stderr.txt

Tests/RunCMake/DisallowedCommands/CMP0035-NEW.cmake

Tests/RunCMake/DisallowedCommands/CMP0035-OLD-result.txt

Tests/RunCMake/DisallowedCommands/CMP0035-OLD-stderr.txt

Tests/RunCMake/DisallowedCommands/CMP0035-OLD.cmake

Tests/RunCMake/DisallowedCommands/CMP0035-WARN-result.txt

Tests/RunCMake/DisallowedCommands/CMP0035-WARN-stderr.txt

Tests/RunCMake/DisallowedCommands/CMP0035-WARN.cmake

Tests/RunCMake/DisallowedCommands/RunCMakeTest.cmake