build: cleanup SSP_SUPPORT configure option

build: cleanup SSP_SUPPORT configure option

Configure variable SSP_SUPPORT is ambiguous for packages (tor, openssh,
avahi, freeswitch). It means 'toolchain supporting SSP', but for toolchain
and depends it means 'build gcc with libssp'.

Musl no longer uses libssp (1877bc9d8f), it has internal support, so
SSP_SUPPORT was disabled leading some package to not use SSP.

No information why Glibc and uClibc use libssp, but they may also provide
their own SSP support. uClibc used it own with commit 933b588e25 but it was
reverted in f3cacb9e84 without details.

Create an new configure GCC_LIBSSP and automatically enable SSP_SUPPORT
if either USE_MUSL or GCC_LIBSSP.

Signed-off-by: Julien Dusser <julien.dusser@free.fr>