title: Defining Variable in Custom Target for CMake
body: <p>I have a CMake file with two targets (say, target1 and target2) defined via "add_custom_target" at the top level of my source directory. I have some external projects in lower level directory. In the extneral project, a "TEST_COMMAND ${Test_Command_Variable}" is defined as part of an ExternalProject_Add(). </p>
  
  <p>I would like to change that vaiarble ${Test_Command_Variable} depending on whether I am using custom target1 or target2. Currently, ${Test_Command_Variable} is defined at the top level CMakeLists.txt before either custom targets are defined. I simply want to vary that variable depending on whether target1 or target2 is called. Is there any way to redefine that variable? Maybe do a conditional if statement dependent on whether target1 or target 2 is chosen (this seems like a trivial thing, but I can't find the way to access the "name" of the custom target!). </p>
  
  <p>To clarify: I have two collections of tests. I want to type "make target1" and it runs my first collection of tests. I want to type "make target2" and it tests my second collection of tests. The problem is I also have an external project, where some of the tests are there. The external projects have a TEST_COMMAND(test_command_variable) that does not differentiate between target1 tests and target2 tests. I would like to be able to change that variable depending on whether I run "make target1" or "make target2".</p>