Fixed compilation issue on some Operating System

Some of us are facing compilation issue on some operating system due to the inclusion of SSH Tunnel code. I have tried a lot to figure it out, but not found the exact cause of the problem. 

Thanks to "Tim Wood" from salesforce for finding the cause of the problem. The problem is order of libssh2 dependent libraries is not correct in linker command. 

Resolution for the issue is to add the libssh2 dependent libraries in LIBS flags instead of LDFLAGS, because LDFLAGS were added ahead of the pgadmin3 objects and LIBS are added after, so because of that linker is able to find all the references from the dependent libraries.

Attached is the patch file to fix the issue. Please review it and if it looks good to you can you please commit it. 

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