build gcc 5.4.0

To enable KASAN, we must use gcc 5.

# wget ftp://ftp.gnu.org/pub/gnu/gcc/gcc-5.4.0/gcc-5.4.0.tar.bz2# tar jxf gcc-5.4.0.tar.bz2# cd gcc-5.4.0# ./contrib/download_prerequisites# mkdir gcc-build# cd gcc-build# ../configure --prefix=/usr/local        \             --disable-multilib         \             --enable-languages=c,c++   \             --enable-libstdcxx-threads \             --enable-libstdcxx-time    \             --enable-shared            \             --enable-__cxa_atexit      \             --disable-libunwind-exceptions     \             --disable-libada                   \             --host x86_64-redhat-linux-gnu     \             --build x86_64-redhat-linux-gnu    \             --with-default-libstdcxx-abi=gcc4-compatible# make -j 32# make install# which gcc/usr/local/bin/gcc# gcc --versiongcc (GCC) 5.4.0Copyright (C) 2015 Free Software Foundation, Inc.This is free software; see the source for copying conditions.  There is NOwarranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.