If using GCC ensure we have at least version 4.7

GCC 4.7 fixes a C++11 issue with `std::packaged_task`. See
<http://stackoverflow.com/a/10940166/939125>.
Customise the feedback provided by `ctb-tile`

By default `ctb-tile` outputs progress using the standard GDAL
progress meter.  Specifying `--quiet` will output nothing except
errors and specifying `--verbose` will show details of the tiles being
created.