cmake : enable some basic warnings globally (#482)

cmake : enable some basic warnings globally (#482)

* cmake : make -Werror=vla global

* cmake : make -Wuninitialized global (part of -Wall)

* tests : fix some -Wunused warnings

This flag is not enabled by default. There are still some warnings
remaining.

* cmake : make -Wsign-compare global (part of -Wall)

* cmake : make -Wall global (minus -Wunused)

* cmake : make -Wstrict-prototypes global

* cmake : add -Wpedantic -Wformat=2 globally

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Co-authored-by: Georgi Gerganov <redacted>