gcc: Environment implementation

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1 4.2 Environment
1 ===============
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1 The behavior of most of these points are dependent on the implementation
1 of the C library, and are not defined by GCC itself.
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1    * 'The mapping between physical source file multibyte characters and
1      the source character set in translation phase 1 (C90, C99 and C11
1      5.1.1.2).'
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11      ⇒Implementation-defined behavior (cpp)Implementation-defined
      behavior.
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