Ubuntu – How to build older gcc versions? Errors about crti.o not being found

It's likely you need to install the libc6-dev package. The errors you see indicate the linker (ld) is unable to link against the C library (and the object files it mentions, crt1.o for example, are part of this package).

I suggest you do sudo apt-get install build-essential, it will install, well, the essential tools and packages for basic builds to work. build-essential is a meta-package, which doesn't install any files of its own but depends on the required packages, so it's a safe bet to install it.

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