CMake remove escape character from generator expresion - cmake

Question

I have the generator expression GEN_EXPR_INCLUDE_PATHS that provides a list of the directories to include.
The goal is to pass this list directly as compiler option via add_custom_command:
add_custom_command(
OUTPUT ${CMAKE_BINARY_DIR}/${HEADER_FILE_NAME}.gch
DEPENDS ${HEADER_FILE} IMPLICIT_DEPENDS CXX ${HEADER_FILE}
COMMAND ${CMAKE_CXX_COMPILER}
${FLAGS}
${HEADER_INCLUDE_PATHS}
${gccGarbageCollectorOpts}
-c ${HEADER_FILE} -o
${CMAKE_BINARY_DIR}/${HEADER_FILE_NAME}.gch
)
Where HEADER_INCLUDE_PATHS is following generator expression:
$<1:-I$<JOIN:$<TARGET_PROPERTY:Some_Target,INTERFACE_INCLUDE_DIRECTORIES>, -I>>
Just because the INTERFACE_INCLUDE_DIRECTORIES of Some_Target is received as generator expression.
But cmake adds escape chars before spaces, and gcc fails due to these extra chars.
Is it possible to do something with it?
I'll be appriciate for any support.

Answer 1

add_custom_command accepts COMMAND parameters as a list. In CMake a list is a string which elements are separated by semicolon, so it is semicolon which should be used for joining, not a space.
Also, for correctly split semicolon-containing strings, add_custom_command needs COMMAND_EXPAND_LISTS keyword.
set(HEADER_INCLUDE_PATHS -I$<JOIN:$<TARGET_PROPERTY:mylib,INTERFACE_INCLUDE_DIRECTORIES>,;-I>)
add_custom_command(
...
COMMAND ${CMAKE_CXX_COMPILER}
...
"${HEADER_INCLUDE_PATHS}" # Double quotes are important!
...
COMMAND_EXPAND_LISTS # This option is important too
)
Actually, an example with a space-separated join of include directories
-I$<JOIN:$<TARGET_PROPERTY:INCLUDE_DIRECTORIES>, -I>
is provided in CMake documentation itself. Not sure about their intention for this example.