mainly from: https://gcc.gnu.org/wiki/InstallingGCC
the main steps:
for example: configuring and building gcc-5.3.0 should be as simple as
tar gcc-5.3.0
./contrib/download_prerequisities
cd ..
mkdir objdir
cd objdir
$PWD/../gcc-5.3.0/configure --prefix=/your/install/path/gcc-5.3.0 --enable-languages=c,c++,fortran,go --disable-multilib
make -j 8
make install
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do soft link, I have installed gcc/g++ 4.8.4
duplicate the original link, for me: sudo mv /usr/bin/gcc /usr/bin/gcc.back.4.8.4
sudo mv /usr/bin/g++ /usr/bin/g++.back.4.8.4
build the new link, for me: sudo ln -s /your/install/path/gcc-5.3.0/bin/gcc /usr/bin/gcc
sudo ln -s /your/install/path/gcc-5.3.0/bin/g++ /usr/bin/g++