Windows uses ;
as a path separator whereas nix (including OSX) uses :
.
The answer is to use the PATH_SEPARATOR
constant:
set_include_path ( get_include_path () . PATH_SEPARATOR . dirname ( __FILE__ ) . '/../lib/propel/runtime/lib/' );
From the docs:
Making use of the PATH_SEPARATOR constant, it is possible to extend the include path regardless of the operating system.