To build uClibc,

1) cp ./extra/Configs/Config.<your arch> ./Config
2) Edit ./Config for your setup.
3) make
4) As root, make install

To compile programs with uClibc,

export PATH={uClibc DEVEL_PREFIX/ROOT_DIR}/bin:$PATH

and then just ./configure and make as usual.

Note: You may also want to look at extra/gcc-uclibc/gcc-uclibc.c for
      additional information concerning what options and environment
      variables the gcc wrapper handles.