Enable incremental linking in debug configurations of WelsDecPlus

This avoids the following warning when linking:
LNK4075: ignoring '/EDITANDCONTINUE' due to '/INCREMENTAL:NO' specification
Don't pass /LTCG to the linker for the console front end binary

The object files for this tool aren't built with the /GL flag
(Whole Program Optimization), leading to the following warning
when linking:
/LTCG specified but no code generation required; remove /LTCG from
the link command line to improve linker performance

Enabling link time code generation for the console front end binary
isn't of much use, since this tool in itself isn't performance
critical.
Remove an unused global variable and commented out code
Merge pull request #248 from mstorsjo/msvc-proj-cleanup

MSVC project file warning cleanup