PR preprocessor/25717
	* init.c (cpp_init_builtins): If __STDC__ will not change value
	between system headers and other sources, define it as a normal
	macro rather than a builtin.
	* macro.c (_cpp_builtin_macro_text) <case BT_STDC>: Only check
	cpp_in_system_header condition.

From-SVN: r110144