complex: make _Complex_I work with gcc -std=c99 -pedantic-errors

include/complex.h

@@ -6,7 +6,11 @@ extern "C" {
 #endif
 
 #define complex _Complex
+#ifdef __GNUC__
+#define _Complex_I (__extension__ 1.0fi)
+#else
 #define _Complex_I 1.0fi
+#endif
 #define I _Complex_I
 
 double complex cacos(double complex);