How to use a combined gcc/binutils/newlibc source tree to create a cross compiler:

Download into a empty directory (${HOME}/tarballs)
binutils - http://gnuftp.uib.no/binutils
gcc - http://gnuftp.uib.no/gcc/
gmp (dependency for gcc) - http://gnuftp.uib.no/gmp/
mpfr (dependency for gcc) - http://gnuftp.uib.no/mpfr/
mpc (dependency for gcc) - http://gnuftp.uib.no/mpc/
isl (OPTIONAL dependency for gcc) http://isl.gforge.inria.fr/


Create yet another directory, enter it and unpack
$ mkdir ${HOME}/src; cd ${HOME}/src
$ for i in ${HOME}/tarballs/*; do tar xf $i; done

Create combined source tree
$ cd gcc-*
$ ln -s ../gmp-* gmp
$ ln -s ../mpfr-* mpfr
$ ln -s ../mpc-* mpc
$ ln -s ../isl-* isl

$ for i in bfd binutils gas ld opcodes; do ln -s ../binutils-*/$i; done

for newlib
$ ln -s ../newlib-*/newlib 

(optional, but when to use it?)
$ ln -s ../newlib-*/libgloss

Build the cross compiler
First, figure out which target you need and that it is available for newlib/gcc
and do something like this (adjust your configure options for your needs).
You might even speed up the compilation if obj/ is on tmpfs.

$ mkdir ${HOME}/obj; cd ${HOME}/obj
$ (TARGET=m68k-elf && ${HOME}/src/gcc-*/configure --prefix=/${TARGET} --libexecdir=/${TARGET}/lib --target=${TARGET} --enable-languages=c,c++ --disable-libstdcxx-pch --with-newlib)

(optional: fix compiler flags)
sed -i 's/\-g /-pipe /' Makefile
sed -i '/^CFLAGS / s/$/ -march=native/' Makefile
sed -i '/^CXXFLAGS / s/$/ -march=native/' Makefile
sed -i '/^CFLAGS_FOR_BUILD/ s/$/ -march=native/' Makefile
sed -i '/^CXXFLAGS_FOR_BUILD/ s/$/ -march=native/' Makefile

Compile and install into staging area
$ make -j$(nproc)
$ make -j2 install-strip DESTDIR=$(pwd)/dest

Now you can move $(pwd)/dest/${TARGET} where ever you want.

If you are sparse on disk space you can trim it a bit as well 

$ rm -rf dest/*/{include,share} dest/*/lib/libcc*
$ find dest -name '*.la' -delete
$ find dest -name "*.a" -exec ./dest/*/bin/*-strip -g "{}" \;

Notes

BSD and MacOS.
This will most probably work if you use gmake instead of make.

Git
If you get your sources with git, you'll also need flex, bison and makeinfo (texinfo).

GMP, MPFR, MPC and ISL
You can skip this if you already have them installed. They are not target libraries.
Keep in mind that you need both the libraries and the headers installed in order to use them. In some distributions that means
that you also need to have the -dev or -devel packages installed.