Rename build options

The Meson developers have recommended that we omit enable/disable
prefixes from option names, as it's redundant with the boolean value.

Note that we intentionally retain underscores, rather than hyphens.
There doesn't seem to be a convention for Meson, but this is suggested
by the Meson documentation, looks nicer, and matches the convention in
CMake world, which is the only other major build system to use -D
notation for defining options.