[CMake] Appending paths to a variable

[Robert Bielik]

I have a structure where I include all subdirs via FILE(GLOB.. and FOREACH
and within each subdir CMakeLists.txt I want to add a path to be used with
INCLUDE_DIRECTORIES in the main CMakeLists.txt

I tried SET(INCL_DIRS ${INCL_DIRS} ${CMAKE_CURRENT_SOURCE_DIR}/include) but the
INCL_DIRS variable is empty when I get "back" to the main CMakeLists.txt file...

How to?
TIA
/R