meson: add summary() if Meson version is new enough

Because we can.

Looks something like this:

orc 0.4.31

  Backends
                 SSE: yes
                 MMX: yes
                NEON: yes
                MIPS: yes
                c64x: yes
             Altivec: yes

  Build options
               Tools: yes
               Tests: yes
            Examples: yes
          Benchmarks: yes
       Documentation: no (disabled)
    orc-test library: yes