Set warning level to 0

[peter-urban]

Related to #77

The reason why the meson test suit produces more errors than the makefile, is that the warning level was set to 2 (my fault). Explanation on meson warning levels:

Warning level 1 flags: -Wall
Warning level 2 flags: -Wall -Wextra
Warning level 3 flags:: -Wall -Wextra -Wpedantic

It may be good to discuss the desired warning flags in a future issue, but for now meson.build should just reproduce the same results as the makefile

[efiring]

Is there also an option for meson to provide more verbose (but not too verbose...) output?
Specifically, with make one can see the underlying command lines being run (e.g., the
gcc invocation with all the defines, flags, input files, etc. expanded out); I assume meson (or actually ninja)
is issuing similar command lines. It seems like it would be helpful for debugging to see them.
Or am I missing something about how meson works?