added CMakeLists.txt

Hi, I wrote CMake config file for glew 1.11.0.
If you will please use this and improve it.
This config file thing could be done in a much nicer way.
Please look at http://www.cmake.org/cmake/help/v3.0/command/configure_file.html


example:
```
CONFIGURE_FILE(glew.pc.in glew.pc @ONLY)
INSTALL(FILES 
         ${CMAKE_BINARY_DIR}/glew.pc
         DESTINATION lib/pkgconfig
)
```