ECW and MrSID support for GDAL

ECW and MrSID are a couple of well-known wavelet compression formats. Unfortunately they are both patented and GDAL support depends on the respective non-free SDKs. The result is that both formats cannot be supported in Debian. Anyway it is still possible to add their support trough plugins for the Debian GDAL binary library, i.e. adding their support without rebuilding the whole beast. This is pure black art, but truly better than rebuild all from sources. First of all, note that ECW and MrSID SDKs are available as source (or binary-only in the case of MrSID) distributions at Erdas and Lizardtech sites; you have to accept their restrictive licenses. Starting from recent version (1.6.3) of GDAL on sid/squeeze it is possible to create the source tarballs of both plugins on fly starting from the GDAL source package. Creating new source package for the plugins from there is straightforward.