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.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@110144 138bc75d-0d04-0410-961f-82ee72b054a4