further gdal nodata handling improvements - refs #2023

Conflicts:
	plugins/input/gdal/gdal_featureset.cpp
tests for #2023
one more test for #2023
enable further test
MinGW support for SConstruct script. Build now properly detects MinGW and finds dependencies correctly.
Properly Initialize mingwbuild variable.
Added singleton unit test to check for multiple instances across multiple dynamic libraries.
Updates for supporting 64bit MinGW build on Windows 7+ via GNU tools.
Final MinGW 64 bit enhancements for Windows 7+ and Server 2008+