GSoC week 10 and 11

symengine-0.1.0 beta version was released this week and these two weeks were spent on making sure symengine works without a problem on Sage.

One issue was the linking of the python libraries in Sage. In binary releases of sage, the variable distutils.sysconfig.get_config_var('LIBDIR')  is wrong. It is set to the build machine's location. In Windows this is set to empty. Earlier, to link the python libraries into the python wrappers, python library was found using the above variable, but in some cases like Sage and Windows this method fails. To fix this, CMake now looks in `sys.prefix`/libs and `sys.prefix`/lib as well to find the python libraries.

Another issue that came up was cmake generating bad link flags. When installing in Sage, it is important to make sure the libraries in Sage are linked and not the system wide libraries. To do that libraries were searched for in the sage directories ignoring the system wide libraries. When given the full path of the libraries to link, we noticed a strange behaviour. `/path/to/sage/local/lib/libgmp.so` was changed to `-lgmp` causing the linker to pick up the system-wide gmp library. 

After reading through CMake documentation, I realized that this was due to `find_library` giving wrong locations of system libraries where there are multiple libraries of the same name for different architectures. For example if the output of `find_library(math NAMES m)` was given to find the standard math library, it may find a `libm.so` that was intended for a different architecture. Therefore when cmake realizes that the library being linked to is a system library then the full path is converted to `-lm` to delegate the task to the linker to find the correct library. 

This behaviour is useful for some scenarios, but in our case, this was not the behaviour I needed. Fortunately there was a workaround for this mentioned in the documentation. Using IMPORTED target feature in CMake, I was able to get CMake to use the full path of the library.

R7 and R8 benchmarks from symbench benchmarks of sage were added to benchmark SymEngine-C++ against GiNaC and also SymEngine-Python against SymPy and Sage.