如何设置gcc5.0与gcc4.9的优先级

环境准备：Ubuntu16，matlab2017（目前只支持gcc4.9）

You could set priority to use [g++/gcc-4.9] before [g++/gcc-5] .

Follow these steps:

--> Make sure gcc-4.9 and g++-4.9 are installed.
~$ sudo apt-get install gcc-4.9 g++-4.9

--> Set "priority=100" for gcc-4.9 and "priority=50" for gcc-5.
~$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.9 100
~$ sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5.0 50

--> Set "priority=100" for g++-4.9 and "priority=50" for g++-5.
~$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.9 100
~$ sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5.0 50

--> Verify the priority settings using:
~$ update-alternatives --query gcc

Name: gcc
Link: /usr/bin/gcc
Status: auto
Best: /usr/bin/gcc-4.9
Value: /usr/bin/gcc-4.9

Alternative: /usr/bin/gcc-4.9
Priority: 100

Alternative: /usr/bin/gcc-5
Priority: 50

~$ update-alternatives --query g++

Name: g++
Link: /usr/bin/g++
Status: auto
Best: /usr/bin/g++-4.9
Value: /usr/bin/g++-4.9

Alternative: /usr/bin/g++-4.9
Priority: 100

Alternative: /usr/bin/g++-5
Priority: 50
参考https://github.com/vlfeat/matconvnet/issues/967