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This also fully honors the PYTHON_VERSION environment variable.

This enables support for older versions of GCC (>=4.2), by falling back
to the boost tr1 implementation when the GCC version is less than 4.4.

This also cleans up some m4 boost macros, and includes other small
corrections to configure.ac.

This allows compilation with older versions of boost (>=1.38), which in
some sistems are the only option. This commit also removes the linking
of the boost_graph shared library, since none of its symbols are
actually required.

It is necessary to use -frounding-math to avoid run-time errors with
certain versions of CGAL.

This also includes the library CGAL as a dependency.