Bingo!!
it should be EXPORTed!
Code:
$ export CALIBRE_DEVELOP_FROM=/home/katsu/calibre/calibre/src
$ /opt/calibre/calibre-debug -g
Hello, world!
Compiling form /home/katsu/calibre/calibre/src/calibre/gui2/shortcuts.ui
Compiling form /home/katsu/calibre/calibre/src/calibre/gui2/filename_pattern.ui
...
Compiling form /home/katsu/calibre/calibre/src/calibre/gui2/catalog/catalog_csv_xml.ui
Starting up...
Started up in 3.13954114914
Thank you so much kovid, chaley, user_none!