RPM based
# yum -y install glibc-devel.i386 libstdc++-devel.i386
# yum -y install glibc-devel.i686 glibc-devel ibstdc++-devel.i686
# zypper in glibc-devel-32bit
debian
$ sudo apt-get install g++-multilib libc6-dev-i386
Example
$ gcc -m32 -o output32 hello.c