Allow duplicate custom targets to be created.
Normally CMake requires that all targets built in a project have
globally unique logical names (see policy
This is necessary to generate meaningful project file names in
Xcode and Visual Studio Generators IDE
generators. It also allows the target names to be referenced
Makefile generators are capable of supporting duplicate
names. For projects that care only about Makefile Generators and do
not wish to support
Xcode or Visual Studio Generators IDE
generators, one may set this property to
to allow duplicate custom targets. The property
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.