[ruby-core:41078] mkmf sets CFLAGS to contain flags that it shouldn't be setting

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Hi!

Mkmf now sets CFLAGS to contain the optflags, debugflags, and
warnflags that Ruby was compiled with.  This is, to me, completely
wrong.  Mkmf has no business telling me what warnings, optimizations,
or debugging settings I should be using.  And, even if it had, it
messes up things when I then add other flags to $CFLAGS that may alter
the interpretation of those flags, for example, -std=c99.

Why was this change made?