Policy CMP0026 is not set: Disallow use of the LOCATION target property - cmake

Question

I have a cmake project with a bunch of different targets in it. First it builds an executable that is used to process some data files (lets call this DataProcessor). Then it processes those data files with the executable. Then, a second executable (we'll call this MyApp) is built and it runs with the processed data files.
There is also a target that takes all the processed data files and MyApp and bundles them up in a tar file so we can distribute them.
In my CMakeLists.txt file there I have the following lines:
get_target_property(DATA_PROCESSOR_EXE DataProcessor LOCATION)
get_target_property(MY_APP_EXE MyApp LOCATION)
I need these to run various other commands in my CMakeLists file. For example, the DATA_PROCESSOR_EXE is used in the custom command to process the data files, like so:
add_custom_command(
OUTPUT ${DATA_OUT}
COMMAND ${DATA_PROCESSOR_EXE} -o ${DATA_OUT} ${DATA_IN}
DEPENDS DataProcessor)
And when I bundle everything up I use MyApp's location as well:
# Convert executable paths to relative paths.
string(REPLACE "${CMAKE_SOURCE_DIR}/" "" MY_APP_EXE_RELATIVE
"${MY_APP_EXE}")
string(REPLACE "${CMAKE_SOURCE_DIR}/" "" DATA_PROCESSOR_EXE_RELATIVE
"${DATA_PROCESSOR_EXE}")
# The set of files and folders to export
set(EXPORT_FILES
assets
src/rawdata
${MY_APP_EXE_RELATIVE}
${DATA_PROCESSOR_EXE_RELATIVE})
# Create a zipped tar of all the necessary files to run the game.
add_custom_target(export
COMMAND cd ${CMAKE_SOURCE_DIR} && tar -czvf myapp.tar.gz ${EXPORT_FILES}
DEPENDS splat
WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}/..)
The problem is that trying to get the target's location property now causes a warning saying:
CMake Warning (dev) at CMakeLists.txt:86 (get_target_property):
Policy CMP0026 is not set: Disallow use of the LOCATION target property.
Run "cmake --help-policy CMP0026" for policy details. Use the cmake_policy
command to set the policy and suppress this warning.
The LOCATION property should not be read from target "DataProcessor".
Use the target name directly with add_custom_command, or use the generator
expression $<TARGET_FILE>, as appropriate.
I don't understand how it wants me to use add_custome_command or what it means by use the generator expression $<TARGET_FILE>

Answer 1

add_custom_command knows about target names. You don't need to fetch the location yourself.
Replace code like
add_custom_command(
OUTPUT ${DATA_OUT}
COMMAND ${DATA_PROCESSOR_EXE}
with code like
add_custom_command(
OUTPUT ${DATA_OUT}
COMMAND DataProcessor
ie, use the name of the target.
The final location of the output executables is not known while CMake is running over your imperative code at 'configure time'. However, add_custom_target can be told to create build rules with content which is only known at generate-time (after configure-time). The way to do that is to use a generator expression.
http://www.cmake.org/cmake/help/v3.0/manual/cmake-generator-expressions.7.html#informational-expressions
Use $<TARGET_FILE_DIR:MyApp> instead of ${MY_APP_EXE_RELATIVE} in your add_custom_target call, for example.