Automatically find libjpeg-turbo headers and libraries

[BobbyCephy]

On Windows in a base conda environment with libjpeg-turbo I only succeed in installing jpegtran-cffi with the following command:
pip install git+https://github.com/jbaiter/jpegtran-cffi.git --global-option="build_ext" --global-option="-I%USERPROFILE%\anaconda3\Library\include" --global-option="-L%USERPROFILE%\anaconda3\Library\lib"

It should install easily on all platforms without any options if a compatible compiler and the libjpeg-turbo package are present.

If the jpegtran executable is found in the PATH, the relative directories of the headers and libraries of libjpeg-turbo can be found automatically.

Related to #20 and #26.