problem in compiling gcc6.4.0 using gcc8.1.1

[C.J.]

Dear specialists,

I tried to install gcc6.4.0 in Fedora28, while something wrong happened as
following.

    In file included from ../.././libgcc/unwind-dw2.c:401:0:
    ./md-unwind-support.h: In function ‘x86_64_fallback_frame_state’:
    ./md-unwind-support.h:65:47: error: dereferencing pointer to incomplete
type ‘struct ucontex_t’
           sc = (struct sigcontext *) (void *) &uc_->uc_mcontext;
                                                   ^~

And at the end the message is here:

    make[3]: *** [../.././libgcc/shared-object.mk:14: unwind-dw2.o] Error 1
    make[3]: *** Waiting for unfinished jobs....
    make[3]: Leaving directory
'/home/rx/program/gcc-6.4.0/x86_64-pc-linux-gnu/libgcc'
    make[2]: *** [Makefile:17378: all-stage1-target-libgcc] Error 2
    make[2]: Leaving directory '/home/rx/program/gcc-6.4.0'
    make[1]: *** [Makefile:23677: stage1-bubble] Error 2
    make[1]: Leaving directory '/home/rx/program/gcc-6.4.0'
    make: *** [Makefile:914: all] Error 2

I have try the previous solution(
https://stackoverflow.com/questions/46999900/how-to-compile-gcc-6-4-0-with-gcc-7-2-in-archlinux)
, but it still failed.
Am I missing something? Thank you!

PS: This problem is also posted in stackoverflow(
https://stackoverflow.com/questions/52498431/compile-gcc6-4-0-using-gcc8-1-1
)

Best regards!
Jianhao