Syntastic won't load checkers on startup

Question

I'm trying to setup Syntastic for C/C++ syntax checking with GCC. GCC is installed and available in the PATH. Here is the syntastic configuration from my .vimrc:

set statusline+=%#warningmsg#
set statusline+=%{SyntasticStatuslineFlag()}
set statusline+=%*

let g:syntastic_always_populate_loc_list = 1
let g:syntastic_auto_loc_list = 1
let g:syntastic_check_on_open = 1
let g:syntastic_check_on_wq = 0

let g:syntastic_c_checkers = ['gcc']
let g:syntastic_cpp_checkers = ['gcc']

Nothing too spicy, the recommended base configuration and two variables defining gcc as the default checker for C and C++ files. But when I open a C/C++ file in vim, nothing happens. :SyntasticInfo yields no currently enabled checker. However, if I manually type in vim :let g:syntastic_c_checkers = ['gcc'], it begins to work. What's happening?

Thank you!