Allow duplicate custom targets to be created.

Normally CMake requires that all targets built in a project have globally unique logical names (see policy CMP0002). This is necessary to generate meaningful project file names in Xcode and VS IDE generators. It also allows the target names to be referenced unambiguously.

Makefile generators are capable of supporting duplicate custom target names. For projects that care only about Makefile generators and do not wish to support Xcode or VS IDE generators, one may set this property to true to allow duplicate custom targets. The property allows multiple add_custom_target command calls in different directories to specify the same target name. However, setting this property will cause non-Makefile generators to produce an error and refuse to generate the project.