cmake_minimum_required (VERSION 3.0.0)

project(MyProject CXX)

set(EXTENSION .c) # Fails
# set(EXTENSION .cpp) # Fails
# set(EXTENSION .txt) # Works

set( TEST_A test )
set( TEST_B ${TEST_A}${EXTENSION} )

get_source_file_property(excluded ${TEST_B} HEADER_FILE_ONLY)
message("Exclusion of ${TEST_B} is: ${excluded}")
get_source_file_property(excluded ${TEST_A} HEADER_FILE_ONLY)
message("Exclusion of ${TEST_A} is: ${excluded}")

set_source_files_properties(${TEST_A} PROPERTIES HEADER_FILE_ONLY FALSE )
set_source_files_properties(${TEST_B} PROPERTIES HEADER_FILE_ONLY TRUE )

get_source_file_property(excluded ${TEST_B} HEADER_FILE_ONLY)
message("Exclusion of ${TEST_B} is: ${excluded}")
get_source_file_property(excluded ${TEST_A} HEADER_FILE_ONLY)
message("Exclusion of ${TEST_A} is: ${excluded}")

if( NOT "${excluded}" STREQUAL "FALSE" )
	message(FATAL_ERROR "\${excluded} is ${excluded} but FALSE was expected.")
endif()