[gta-list] Improvements to configure

[Volker Froehlich]

Hello,

here's a small list of findings in the configure script. They're mostly
"nice to have".

* The pcl pkconfig file is called pcl_io in Fedora.
  Configure expects it to be called pcl_io-1.6 though.

* If g++ is not present, configure reports:
  "GCC atomic builtins not found. Please use a newer compiler."

  This is a bit misleading, I think.

* The dcmtk library is in a sub-directory in Fedora.
  So are the headers of netpbm. Having CFLAGS and LIBS variables
  would be handy.

Greetings,

Volker