Always define __GLASGOW_HASKELL_PATCHLEVEL1/2__ macros

As #21076 (closed) 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 (closed)