grib2 support assumes libpng15

[opoplawski]

On Fedora, grib2 is compile with libpng and there is no libpng15, so the grib2 configure test fails:

checking for main in -lpng15... no
...
  |  - GRIB2 disabled             |

[opoplawski]

It would also be helpful to be able to specify the name of the g2c library - on Fedora it's something like g2c_v1.6.3.