macro(LOG line)
   message(STATUS ${line})
   file(APPEND ${CMAKE_CURRENT_BINARY_DIR}/AbsPathTest.txt "${line}\n")
endmacro(LOG line)

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/AbsPathTest.txt "test\n")

set(path1 "/abs/unix")
set(path2 "../rel")
set(path3 ${CMAKE_ROOT})

macro(GET_ABS_PATH _abs_filename _filename)
   if(${_filename} MATCHES "^([a-zA-Z]:)?/.+")
      set(${_abs_filename} ${_filename})
   else(${_filename} MATCHES "^([a-zA-Z]:)?/.+")
      set(${_abs_filename} ${CMAKE_CURRENT_SOURCE_DIR}/${_filename})
   endif(${_filename} MATCHES "^([a-zA-Z]:)?/.+")
endmacro(GET_ABS_PATH)

get_abs_path(abs ${path1})
log(${path1})
log(${abs})

get_abs_path(abs ${path2})
log(${path2})
log(${abs})

get_abs_path(abs "${path3}")
log(${path3})
log(${abs})