CentOS6.5默认安装的gcc版本是4.4.7，而处于某些软件的编译需求，gcc版本要求在5.0以上，因此只能DIY了。
1. 环境准备
我自己本地配了个CentOS6.5的虚机，配置好yum源（方法参考：https://www.jianshu.com/p/422813bdd34c）。
- 编译gcc之前，首先需要环境中已经安装一套gcc和g++
yum install gcc
yum install gcc-c++
- 下载gcc5.1.0源码：http://mirrors.concertpass.com/gcc/releases/gcc-5.1.0/
- 上传服务器解压
tar -xzvf gcc-5.1.0.tar.gz
cd gcc-5.1.0
2. 下载依赖
gcc的源码中有个依赖的下载脚本，如果虚机网络可以正常访问外网，可以直接执行脚本
cat ./contrib/download_prerequisites
GRAPHITE_LOOP_OPT=yes
# Necessary to build GCC.
MPFR=mpfr-2.4.2
GMP=gmp-4.3.2
MPC=mpc-0.8.1
...
首先执行下载脚本，报错，DNS解析有问题
[root@localhost contrib]# ./download_prerequisites
--2018-06-15 09:59:24-- ftp://gcc.gnu.org/pub/gcc/infrastructure/mpfr-2.4.2.tar.bz2
=> “mpfr-2.4.2.tar.bz2”
Resolving gcc.gnu.org... failed: Name or service not known.
wget: unable to resolve host address “gcc.gnu.org”
尝试解决，但是我的虚机是公司内网，无奈只能手动下载依赖的包了
vim /etc/resolv.conf
nameserver 8.8.8.8 #google域名服务器
nameserver 8.8.4.4 #google域名服务器
下载下来的依赖包放到gcc源码的contrib目录下，然后编辑download_prerequisites脚本，把下载的命令全部注释掉，重新执行一次。
3. 编译安装
第一次编译报错
./configure --prefix=/usr/gcc --enable-languages=c,c++
...
configure: error: Building GCC requires GMP 4.2+, MPFR 2.4.0+ and MPC 0.8.0+.
Try the options to specify
their locations. Source code for these libraries can be found at
their respective hosting sites as well as at
ftp://gcc.gnu.org/pub/gcc/infrastructure/. See also
http://gcc.gnu.org/install/prerequisites.html for additional info. If
you obtained GMP, MPFR and/or MPC from a vendor distribution package,
make sure that you have installed both the libraries and the header
files. They may be located in separate packages.
添加--with-gmp --with-mpfr --with-mpc之后再次尝试，报同样的错误，似乎gcc并不会自动编译安装这些依赖包，尝试先手动安装。
3.1 编译安装GMP
cd /opt/gcc-5.1.0/contrib/gmp
./configure --prefix=/usr/gmp-4.3.2
make && make install
3.2 编译安装MPFR
cd /opt/gcc-5.1.0/contrib/mpfr-2.4.2
./configure --prefix=/usr/mpfr-2.4.2 --with-gmp=/usr/gmp-4.3.2
make && make install
3.3 编译安装MPC
cd /opt/gcc-5.1.0/contrib/mpc-0.8.1
./configure --prefix=/usr/mpc-0.8.1 --with-gmp=/usr/gmp-4.3.2 --with-mpfr=/usr/mpfr-2.4.2
make && make install
然后配置lib环境变量
export LD_LIBRARY_PATH=/usr/gmp-4.3.2/lib:/usr/mpc-0.8.1/lib/:/usr/mpfr-2.4.2/lib:$LD_LIBRARY_PATH
再次尝试编译gcc
./configure --prefix=/usr/gcc --enable-languages=c,c++ --with-gmp=/usr/gmp-4.3.2 --with-mpfr=/usr/mpfr-2.4.2 --with-mpc=/usr/mpc-0.8.1
...
checking for compatible ISL... no
*** This configuration is not supported in the following subdirectories:
target-libmpx gnattools gotools target-libada target-libgfortran target-libgo target-libffi target-libbacktrace target-zlib target-libjava target-libobjc target-liboffloadmic target-boehm-gc
(Any other directories should still work fine.)
checking for default BUILD_CONFIG... bootstrap-debug
checking for --enable-vtable-verify... no
/usr/bin/ld: crt1.o: No such file: No such file or directory
collect2: ld returned 1 exit status
configure: error: I suspect your system does not have 32-bit developement libraries (libc and headers). If you have them, rerun configure with --enable-multilib. If you do not have them, and want to build a 64-bit-only compiler, rerun configure with --disable-multilib.
缺少32位的依赖包，但是我只想编译64位的，因此添加--disable-multilib编译选项，再次尝试，成功。最后一步就是make和make install了。比较漫长。。。
