build-sys: disable _FORTIFY_SOURCE for -Og builds for now

Older gcc versions throw things like:

  In file included from /usr/include/fcntl.h:302:0,
                 from ../src/core/execute.c:25:
In function 'open',
    inlined from 'open_null_as' at ../src/core/execute.c:196:12:
/usr/include/bits/fcntl2.h:50:24: error: call to '__open_missing_mode'
  declared with attribute error: open with O_CREAT in second argument needs 3 arguments
    __open_missing_mode ();