* add ability to disable build make relative path

https://github.com/conda/conda-build/issues/14




sometimes a package builder might want to set no rpath or a own one for internal usage.

it would be nice if one could have an option to disable the mk_relative rpath part.

build example something like
```
#CHECK: BUILD_NO_RPATH
if [ "${BUILD_NO_RPATH}" != "y" ]; then
    echo 'this recipes needs conda config: build_no_rpath: True'
    exit 1
fi

unset LD_RUN_PATH
./bootstrap.sh
LDFLAGS="-Wl,-rpath,${PREFIX}/usr" \
./configure --prefix=${PREFIX}
make
make install
```