[CMake] What so special about .exe?

[Eric Lemings]

Funny.  The following simple CMakeLists.txt file works as expected.

    add_custom_command (OUTPUT Foo.txt
        COMMAND echo Foo > ${CMAKE_CURRENT_BINARY_DIR}/Foo.txt VERBATIM)
    add_custom_target (Foo ALL DEPENDS Foo.txt)

Changing the .txt suffix to .exe however produces the following error
(on Windows):

    NMAKE : fatal error U1073: don't know how to make 'Foo.exe'

What so special about .exe?

Thanks,
Eric.