Add support for building outside of Python environments

[joerowell]

Adds -fPIC to allow building outside of Python environments. Before:

[100%] Linking CXX shared library libMHCUDA.so
/usr/bin/ld: CMakeFiles/MHCUDA.dir/MHCUDA_generated_kernel.cu.o: warning: relocation against `_Z21weighted_minhash_cudaPKfS0_S0_S0_PKjS2_PKiijjPj' in read-only section `.text.startup'
/usr/bin/ld: CMakeFiles/MHCUDA.dir/MHCUDA_generated_kernel.cu.o: relocation R_X86_64_PC32 against symbol `_Z8log_cudajPf' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
collect2: error: ld returned 1 exit status

After:

[100%] Linking CXX shared library libMHCUDA.so
[100%] Built target MHCUDA