config: move the "build shared libraries" option to the OS menu

The "Build shared libraries" config option is dependant on the type of
"Target OS".

Moving this options to the "Target OS" sub-menu is also better in the user
perspective: he/she no longer needs to go back and forth to see if he/she
missed any option.

# Overall toolchain configuration: paths, jobs, etc...

menu "Paths and misc options"

source "config/global/ct-behave.in"

source "config/global/paths.in"

source "config/global/download.in"

source "config/global/extract.in"

source "config/global/build-behave.in"

source "config/global/logging.in"

endmenu