ExternalProject_Add - Cannot set DOWNLOAD_NO_EXTRACT to 0/NO/OFF/FALSE

The value set for the DOWNLOAD_NO_EXTRACT property of an externally added project (ExternalProject_Add) is ignored. The mere existence of the property makes CMake skip extraction of the sources, regardless of its value, being set to e.g. 0/NO/OFF or FALSE.

Here is an excerpt of my code, which I hope is enough to understand the problem. Only the part with the exclamation marks should be of interest. I did not delete the other properties since maybe a combination of them or the order could be interesting for debugging purposes?!

# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
# Try to extract/skip the extraction depending on a variable.
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
# Set a variable containing both the property and the value.
# This does not work at all. The value would be appended to the 'URL_HASH'.
set(boost_DOWNLOAD_NO_EXTRACT "DOWNLOAD_NO_EXTRACT true")

# Set a variable containing a boolean value.
#set(boost_DOWNLOAD_NO_EXTRACT_VALUE 0)
#set(boost_DOWNLOAD_NO_EXTRACT_VALUE NO)
#set(boost_DOWNLOAD_NO_EXTRACT_VALUE OFF)
set(boost_DOWNLOAD_NO_EXTRACT_VALUE FALSE)
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

# Add boost as an external project.
ExternalProject_Add(boost

	# -------------------------------------------------------------------------------
	# Override the <STAMP_DIR>, to enable builds in multiple versions!
	# -------------------------------------------------------------------------------
	# Once boost has been built for any address-model (32bit/64bit) for any compiler
	# (msvc, gcc, etc.), it won't be build again for any other address-model or 
	# compiler, because it says in the <STAMP_DIR>, that boost has already been 
	# built. To prevent that behavior, the stamp dir is put into the 'build' 
	# directory into a subfolder called stamp.
	STAMP_DIR stamp

	URL ${boost_URL}
	URL_HASH SHA256=${boost_SHA1}
	
	# Do not extract the sources, if they already exist.
	# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	# NONE OF THE FOLLOWING LINES WORK!
	# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	#${boost_DOWNLOAD_NO_EXTRACT}	
	DOWNLOAD_NO_EXTRACT ${boost_DOWNLOAD_NO_EXTRACT_VALUE}
	#DOWNLOAD_NO_EXTRACT 0
	#DOWNLOAD_NO_EXTRACT NO
	#DOWNLOAD_NO_EXTRACT OFF
	#DOWNLOAD_NO_EXTRACT FALSE
	# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
	
	# Uncomment do debug the configure command.
	#CONFIGURE_COMMAND echo "boost_CONFIGURE_COMMAND ==> ${boost_CONFIGURE_COMMAND}"
	#	COMMAND ${boost_CONFIGURE_COMMAND}
	CONFIGURE_COMMAND ${boost_CONFIGURE_COMMAND}

	# Uncomment do debug the build command.
	#BUILD_COMMAND echo "boost_BUILD_COMMAND ==> ${boost_BUILD_COMMAND}"
	#	COMMAND ${boost_BUILD_COMMAND}
	BUILD_COMMAND ${boost_BUILD_COMMAND}

	INSTALL_COMMAND ""
	
	# Force the external project to always be built.
	# If it is built once in a e.g. 32bit version, a call to build it as 64bit
	# will not be executed, since CMAKE thinks it is already build. Forcing the build
	# solves this problem.
	BUILD_ALWAYS true
	
	# Do not set this true, as it would override the <SOURCE_DIR> to be equal to <BINARY_DIR>
	BUILD_IN_SOURCE false
)

# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
# Get the value
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
ExternalProject_Get_Property(boost DOWNLOAD_NO_EXTRACT)
# Correctly prints the set value! But the value seems to be ignored! For example:
# -- DOWNLOAD_NO_EXTRACT="FALSE"
cmake_print_variables(DOWNLOAD_NO_EXTRACT) 
# !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Tested on Windows 10 with CMake versions:

• cmake version 3.16.0-rc1
• cmake version 3.17.0