Add version variable to doxygen configuration

E.g., `GR1C_VERSION=0.6.3 make doc` will cause the version number
0.6.3 to appear in the Doxygen-generated documentation.  To obtain
this number from the last version tag, try

    $ GR1C_VERSION=$(git tag -l 'v*'|tail -1|cut -d v -f 2) make doc