[Cmake] backwards compatibility problem with CMake1.4

[Ian Scott]

On Unix if you do not specify the file ending for the source file in an
ADD_EXECUTABLE command, the Makefile isn't correct

For example
Replacing
  ADD_EXECUTABLE(test_vasm test_vasm.cxx)
with
  ADD_EXECUTABLE(test_vasm test_vasm)

causes every instance of
   test_vasm.o
in the produced Makefile is replaced with
   /work/vxl/obj/isbe_apm/vasm/tests.o
or just
   tests.o

Make then complains that there is no build rule for test.o. (There is no
file called "tests.cxx" anywhere.)
It is relatively trivial problem to fix, but if you are aiming for backwards
compatibility it might be worth taking a look at. Sorry I didn't spot it
earlier, but my usual platform is MSVC (which doesn't appear to have a
similar problem)
Ian.