Compile as C code via VPC

[Adrianilloo]

Time ago I managed to create a custom SQLite3 driver project which embeds the all-in-one native sqlite3.c amalgamation file, setting it to compile as C code (as it's necessary given it fails with C++ compilers otherwise). I did it using $CompileAs for Windows and a $CustomBuildStep for Linux.

However, the later entry now fails with the updated VPC Linux executable from the "TF2-SDK" update, causing an error in my Azure DevOps CI/CD pipeline (during VPC projects creation):

ERROR: Could not get $CustomBuildStep tool interface from configuration

It seems support for this was removed on Linux, at least. So, I first tried to reuse the $CompileAs directive, originally aimed for Windows, and it made no effect (VPC generated correctly but compilation errors stayed).

For reference, this is my original VPC file.

I saw the VPC sample file for libspng, for reference, and I tried adding $GCC_ExtraCompilerFlags "$BASE -x c" [$POSIX], but it didn't work.

Related: #774