Always define __GLASGOW_HASKELL_PATCHLEVEL1/2__ macros

As #21076 reports if you are using `-Wcpp-undef` then you get warnings
when using the `MIN_VERSION_GLASGOW_HASKELL` macro because
__GLASGOW_HASKELL_PATCHLEVEL2__ is very rarely explicitliy set (as
version numbers are not 4 components long).

This macro was introduced in 3549c952
and it seems the bug has existed ever since.

Fixes #21076