Provide fallback on GHC stage1 compilation if Alex isn't installed on system

Motivation

Provide fallback on GHC stage 1 compilation if Alex isn't installed on system.

That's prevent compilation GHC on MSYS2/MinGW toolchain (because on start of compilation we haven't it in "stage 0 GHC", which packaged to release.

Now we have:

checking for happy... no
checking for version of happy...
checking for alex... no
checking for version of alex...
configure: error: Alex version 3.1.7 or later is required to compile GHC.

Proposal

Provide fallback on GHC stage 1 compilation if Alex isn't installed on system.

This is related to #16780 (closed)

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