为CentOS7编译升级GCC

CentOS7默认的GCC版本是４.８，不支持C++14，对C++11支持也不完善，编译时需要加上-std=c++11才能勉强支持C++11，所以对GCC来个升级。

因为3.10的内核实在太老，加之GCC4.8和最新的GCC8.2代差太大，直接用GCC4.8编译安装GCC7或8是不能成功的，我选择了GCC6.5，6.5对C++11支持很完美，还支持C++14.

• 1、下载和解压源码

这里列出了GCC主要的几个镜像站，很遗憾亚洲只有日本一个，国内一个都没有

https://gcc.gnu.org/mirrors.html

GCC mirror sites

Our releases are available on the GNU FTP server and its mirrors. The following sites mirror the gcc.gnu.org FTP site (Phoenix, Arizona, USA) directly:

• France (no snapshots): ftp.lip6.fr, thanks to ftpmaint at lip6.fr
• France, Brittany: ftp.irisa.fr, thanks to ftpmaint at irisa.fr
• France, Versailles: ftp.uvsq.fr, thanks to ftpmaint at uvsq.fr
• Germany, Berlin: ftp.fu-berlin.de, thanks to ftp at fu-berlin.de
• Germany: ftp.gwdg.de, thanks to emoenke at gwdg.de
• Germany: ftp.mpi-sb.mpg.de, thanks to ftpadmin at mpi-sb.mpg.de
• Germany: http://gcc.cybermirror.org, thanks to Sascha Schwarz (cm at cybermirror.org)
• Greece: ftp.ntua.gr, thanks to ftpadm at ntua.gr
• Hungary, Budapest: robotlab.itk.ppke.hu, thanks to Adam Rak (neurhlp at gmail.com)
• Japan: ftp.tsukuba.wide.ad.jp, thanks to Kohei Takahashi (tsukuba-ftp-servers at tsukuba.wide.ad.jp)
• The Netherlands, Dronten:   http://mirror.koddos.net/gcc/ |   rsync://mirror.koddos.net/gcc/,   thanks to Martin (mirror@koddos.net) at KoDDoS.
• The Netherlands, Nijmegen: ftp.nluug.nl, thanks to Jan Cristiaan van Winkel (jc at ATComputing.nl)
• Russia, Novosibirsk: http://mirror.linux-ia64.org/gnu/gcc/, thanks to Daniel Volchixin <daniel@volchixin.co.uk>
• Slovakia, Bratislava: gcc.fyxm.net, thanks to Jan Teluch (admin at 2600.sk)
• UK: ftp://ftp.mirrorservice.org/sites/sourceware.org/pub/gcc/, thanks to mirror at mirrorservice.org
• US, San Francisco: https://bigsearcher.com/mirrors/gcc/, thanks to info@bigsearcher.com
• US, San Jose: