The symlink to the 4.8.2 directory is nothing to worry about, it's normal for the libstdc++ headers on Red Hat Enterprise Linux (and therefore CentOS) to be arranged like that.

gcc --version will tell you the version of the gcc executable in your path.

rpm -q libstdc++-devel will tell you the version of the package that owns the C++ standard library headers.

rpm -ql libstdc++-devel will list the files installed by that package, which will include the files under /usr/include/c++/4.8.2

rpm --verify libstdc++-devel will check that you haven't messed up the C++ headers by replacing them with something else.

The error is more concerning, that implies you have messed something up. My guess would be it's in the from [...omitted by myself as it is irrelevant] part, which may actually be very relevant. std::locale should be declared in <bits/locale_classes.h> which is included before <bits/locale_facets_nonio.h>, so if it wasn't declared my guess is that you have some header that defines _LOCALE_CLASSES_H and prevents the standard library header from being read. Do not define include guards that start with underscores, they are reserved names.

Major versions add new features. Minor versions fix bugs. If a bug is found in an earlier major version, a new minor version is released to fix it.

First erase the current update-alternatives setup for gcc and g++:

sudo update-alternatives --remove-all gcc 
sudo update-alternatives --remove-all g++

Install Packages

It seems that both gcc-4.3 and gcc-4.4 are installed after install build-essential. However, we can explicitly install the following packages:

sudo apt-get install gcc-4.3 gcc-4.4 g++-4.3 g++-4.4

Install Alternatives

Symbolic links cc and c++ are installed by default. We will install symbol links for gcc and g++, then link cc and c++ to gcc and g++ respectively. (Note that the 10, 20 and 30 options are the priorities for each alternative, where a bigger number is a higher priority.)

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.3 10
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.4 20

sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.3 10
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.4 20

sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30
sudo update-alternatives --set cc /usr/bin/gcc

sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30
sudo update-alternatives --set c++ /usr/bin/g++

Configure Alternatives

The last step is configuring the default commands for gcc, g++. It's easy to switch between 4.3 and 4.4 interactively:

sudo update-alternatives --config gcc
sudo update-alternatives --config g++

Or switch using script:

#!/bin/sh

if [ -z "$1" ]; then
    echo "usage: $0 version" 1>&2
    exit 1
fi

if [ ! -f "/usr/bin/gcc-$1" ] || [ ! -f "/usr/bin/g++-$1" ]; then
    echo "no such version gcc/g++ installed" 1>&2
    exit 1
fi

update-alternatives --set gcc "/usr/bin/gcc-$1"
update-alternatives --set g++ "/usr/bin/g++-$1"

Just type

gcc --version

in any terminal near you.. ;-)

First of all try to find a Ubuntu release that was out after the release of the required version of GCC. You can find the release history of GCC on GCC Releases - GNU Project - Free Software Foundation (FSF) and that of Ubuntu on Ubuntu version history - Wikipedia.

GCC 6.3 was released on December 21, 2016 and the closest Ubuntu release was Ubuntu 17.04 (Zesty Zapus) which was released in April 2017. You can use the archives of Zesty to install that. But since Zesty reached end of life way back in January 2018, therefore, its archives have been moved to Old Releases. To install GCC 6.3 from its repository:

1. Add the repository of Zesty and disable the Universe repository of Bionic since it contains 6.4 as well as 6.5 which might get installed while trying to install 6.3.

   echo "deb http://old-releases.ubuntu.com/ubuntu zesty main" | sudo tee /etc/apt/sources.list.d/zesty.list
   sudo apt-add-repository -r universe
   

2. Update the available package information and install GCC 6.3.

   sudo apt update
   sudo apt install gcc-6
   

3. Add GCC 6 as an alternative for GCC.

   sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 1
   

4. Check the GCC version using gcc -v. You should get the output like:

   Using built-in specs.
   COLLECT_GCC=gcc
   COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
   Target: x86_64-linux-gnu
   Configured with: ../src/configure -v --with-pkgversion='Ubuntu 6.3.0-12ubuntu2' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
   Thread model: posix
   gcc version 6.3.0 20170406 (Ubuntu 6.3.0-12ubuntu2) 
   

5. Once you're done with "making", you can remove the installed GCC 6.3 and the repository information of Zesty. And re-enable Universe repository of Bionic.

   sudo apt purge gcc-6
   sudo apt autoremove --purge
   sudo rm /etc/apt/sources.list.d/zesty.list
   sudo apt-add-repository universe
   

6. Fix the symlink for /usr/bin/gcc.

   ln -sf /usr/bin/gcc-7 /usr/bin/gcc