"git" is obviously needed for pulling the latest source code.

"base-devel" is needed to actually make the compiler toolchain work.

Both packages are not installed in current MSYS2 base installations.

The libepoxy library is required for shader support.

Since the README file was initially written, the total download size for
the msys toolchain including all libraries increased from 500MiB to a
whopping 1100MiB, resulting in an install size of over 6000MiB for each
architecture.

Unfortunately, the wiki claims the contents are out of date, and can't seem to find the latest wiki revision.