Starting with GDB10, it requires support for std::future<> in the
compiler. Such support has not been available on some architectures
until GCC9 (see PR 64735). I haven't determined the exact list of
affected architectures, so decided to make it a broad dependency: for
GDB10+, you need GCC9+.
Signed-off-by: Alexey Neyman <stilor@att.net>
586c2d0156