centos7默认安装gcc4.8.5
    升级gcc
    需要三个依赖：直接wget下载安装
    wget ftp://gcc.gnu.org/pub/gcc/infrastructure/gmp-6.1.0.tar.bz2
    wget ftp://gcc.gnu.org/pub/gcc/infrastructure/mpfr-3.1.4.tar.bz2
    wget ftp://gcc.gnu.org/pub/gcc/infrastructure/mpc-1.0.3.tar.gz

    gmp编译报错： configure:error:no usable m4 in$path or /user/5bin
    解决方案：yum -y install m4
    gcc报错：configure: error: I suspect your system does not have 32-bit developement libraries (libc and header
    解决方案：yum install glibc-devel.i686
    yum install libgcc.i686

    下载gcc-9.1.0.tar.gz
    tar -zxvf gcc-9.1.0.tar.gz
    cd gcc-9.1.0
    ./configure
    make &&make install