Improve build setup



See merge request !15

* Correctly set CFLAGS and CXXFLAGS
* Add setup of LDFLAGS in case it is set
* Prefer environment variables in place of buildout-set variables
* Setup CMAKE_PREFIX_PATH so CMake does find our externals
* Introduce flags for automatic MacOSX compatibility

[extension] Experimental fix for issue #13 (closes #13)

fixes a problem when python site.sys.path contains non-real paths. Also, orders the paths correctly when they are inserted. (#12)

fixes a problem when python site.sys.path contains non-real paths. Also, orders the paths correctly when they are inserted.

This reverts commit efe6c2c2.

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