Add build/install flag --define which passes arguments to cmake

python setup.py build --define="CMAKE_BUILD_TYPE:STRING='Debug'; \
WITH_BFD:BOOL=OFF; WITH_CSYMPY_ASSERT:BOOL=ON"

results in

Configuration results
---------------------
C++ compiler: /usr/bin/c++
Build type: Debug
C++ compiler flags: -std=c++0x -Wall -Wextra -fPIC -g -Wno-unused-parameter -ggdb
Installation prefix: /usr/local
WITH_CSYMPY_ASSERT: ON
WITH_CSYMPY_RCP: yes
WITH_CSYMPY_THREAD_SAFE: no
GMP_INCLUDE_DIRS: /usr/include
GMP_LIBRARIES: /usr/lib64/libgmpxx.so;/usr/lib64/libgmp.so
WITH_BFD: OFF
WITH_PYTHON: yes
Python library: /home/ptb/miniconda3/lib/libpython3.4m.so
Python include path: /home/ptb/miniconda3/include/python3.4m
Python install path: /home/ptb/miniconda3/lib/python3.4/site-packages
Cython path: /home/ptb/miniconda3/bin/cython
WITH_ECM: no
WITH_PRIMESIEVE: no
Merge pull request #266 from pbrady/setup_define

Improve setup.py to pass along arguments to cmake