meson.build: make valgrind optional
Now that we're providing the test suite as installed option, distributions will likely include it as a test package. valgrind is only used for the meson-specifc test setup. So let's make it optional.
Now that we're providing the test suite as installed option, distributions will likely include it as a test package. valgrind is only used for the meson-specifc test setup. So let's make it optional.