Additional optional dependency on librasterlite
always link libdl on linux - needed on some systems after #1804 - closes #1998
Merge pull request #1997 from kapouer/patch-1

Additional optional dependency on librasterlite
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+