cmake: Add ability to pass in GGML_BUILD_NUMBER (#1096)

cmake: Add ability to pass in GGML_BUILD_NUMBER (#1096)

This makes git as a dependency optional, and is useful in the case where
ggml is built not from git, but from a tarball, or a distribution source
package.

This conditional also affects GGML_BUILD_COMMIT. Nothing seems to be
using it, though, so there doesn't seem much value factor it out, or
even require it.