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.

config: make "Build shared libraries" depend on KERNEL_SUPPORTS_SHARED_LIBS

Now that all kernels that support shared libraries select the correct config
option, we can change the dependency rules of SHARED_LIBS accordingly.

config: Linux kernel selectes shared libraries support

The Linux kernel supports using shared libraries, so prompt the user.

config: introduce the KERNEL_SUPPORTS_SHARED_LIBS silent config option

The target OSes will be able to select this option if they support shared
libraries, instead of having "Build shared libaries" depend on each OS.

config: swap the debug and tools sub-menus

ltrace, in the debug sub-menu, selects libelf, in the tools sub-menu.
Inverse the order of the two sub-menus, so that the user does not have
to go back and forth between the two sub-menus.

config: move down companion libraries sub-menu

Move the companion libraries sub-menu down the main menu.
That way, the user does not need to go back and forth in the menu
to change options set by the different components that select the
companion libraries (binutils, gcc, gdb).

companion-libs: PPL/CLooG/MPC require GMP and MPFR

The PPL, CLoog/ppl and MPC libraries require GMP and MPFR.
Select GMP/MPFR appropriately.

tools wrapper: introduce the silent WRAPPER_NEEDED config option

Add the WRAPPER_NEEDED silent config option, that can be selected by
components that require it (companion libs so far).
Rely on this config option when deciding to install the wrapper,
instead of checking GMP/MPFR or PPL/CLoog/MPC.

Merge the C wrapper.

config: choose whether to use the shell or the C wrapper

Offer a config choice on whether to isntall the script wrapper, or
the compiled C wrapper. Update docs/overview.txt accordingly.