$ https://ftp.jaist.ac.jp/pub/GNU/gcc/gcc-9.3.0/gcc-9.3.0.tar.xz
$ tar xvf gcc-9.3.0.tar.xz 
$ ./contrib/download_prerequisites 
$ unset LIBRARY_PATH CPATH C_INCLUDE_PATH PKG_CONFIG_PATH CPLUS_INCLUDE_PATH INCLUDE
$ mkdir build
$ cd build
$ ../configure --prefix=/opt/gcc/9.3.0 --enable-languages=c,c++,fortran --disable-multilib --disable-bootstrap
$ make -j
$ make install

export GCC_ROOT=/opt/gcc-9.3.0
export PATH=${GCC_ROOT}/bin:$PATH
export LD_LIBRARY_PATH=${GCC_ROOT}/lib:$LD_LIBRARY_PATH
export LD_LIBRARY_PATH=${GCC_ROOT}/lib64:$LD_LIBRARY_PATH
export LIBRARY_PATH=${GCC_ROOT}/lib:$LIBRARY_PATH
export LIBRARY_PATH=${GCC_ROOT}/lib64:$LIBRARY_PATH