Replace VS project files with IDE-independent build system.

[WhyIsEvery4thYearAlwaysBad]

Currently the Source SDK uses Visual Studio project files for compilation. However this causes an IDE-dependency, which can cause issues for clients that do not have, or can't buy, Visual Studio.

[Yetoo1]

There is no IDE dependency. Msbuild can compile sln on linux. Instructions to do so on Valve dev wiki here: https://developer.valvesoftware.com/wiki/Source_SDK_2013

[Jan200101]

Not quite correct.

Non Windows builds already create independent build files that are IDE and system independent.

The problem on Windows is that MSVC is the only toolchain with full support and unless you want to go through the effort and trouble to manually assemble all paths and directories its best to simply depend on Visual Studio

[samzanemesis]

VPC can output to multiple formats, and VS Community is free