[CMake] Custom target

[Ian Liu Rodrigues]

> Target names should be unique but if you really want to have all your benchmark targets with the same name look at this variable: ALLOW_DUPLICATE_CUSTOM_TARGETS

I understand why some targets must be unique. But take the "test"
target for instance.

When we add_test we can execute "make test" on the root dir to execute
all tests, but we can also execute "make test" inside the subdirs to
execute only that subdir tests.

Isn't it possible to create something similar for benchmark without
changing the default behavior of CMake regarding duplicate targets?

Regards,
Ian