Unknown toolset: darwin

You can specify the toolset as the argument, i.e.:
    ./build.sh gcc

Toolsets supported by this script are:
    acc, clang, como, gcc, intel-darwin, intel-linux, kcc, kylix, mipspro,
    pathscale, pgi, qcc, sun, sunpro, tru64cxx, vacpp

For any toolset you can override the path to the compiler with the CXX
environment variable. You can also use additional flags for the compiler
with the CXXFLAGS environment variable.

A special toolset; cxx, is available which is used as a fallback when a more
specific toolset is not found and the cxx command is detected. The 'cxx'
toolset will use the CXX, CXXFLAGS, and LIBS environment variables, if present.

Similarly, the cross-cxx toolset is available for cross-compiling by using the
BUILD_CXX, BUILD_CXXFLAGS, and BUILD_LDFLAGS environment variables to compile
binaries that will be executed on the build system. This allows CXX etc. to be
set for cross-compilers to be propagated to subprocesses.