Generate debug symbols with CFLAGS=-g3

[samrose]

Chore

Over here #1195 (comment) @pashkinelfe suggested the following

I'd drop --enable-debug and use CFLAGS=-g3 instead like in fdaee4b

As per
https://wiki.postgresql.org/wiki/Developer_FAQ --enable-debug will also set less speed-optimal optimization i.e. -Og CFLAGS (and also -fno-omit-frame-pointer) that could decrease performance compared to default -02. I think for production system -g3 is better

So this is an issue to create PR that tries to do this + making sure that the new approach is compatible with separateDebugInfo = true; in our postgresql package