With cmake, CFLAGS has to be set when invoking cmake, not make (which totally
ignores the value of CFLAGS when it runs and only keeps the one from cmake).
Also, in that case the flags were either redundant (-Werror etc) or wrong
(-std=c99 -pedantic) as some parts of the library will not build with
-pedantic (see the other -pedantic tests, which are correct, for what needs to
be disabled).
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