- Update premake build script for VS2017+ build support.

- Update premake (for VS2019 support).
- Update Xenia build script to detect VS2017+ version.
- Update Xenia premake scripts due to updated premake.
- Fix cc override in Xenia build script.

premake5.lua

@@ -7,7 +7,7 @@ objdir(build_obj)
 
 -- Define an ARCH variable
 -- Only use this to enable architecture-specific functionality.
-if os.is("linux") then
+if os.istarget("linux") then
   ARCH = os.outputof("uname -p")
 else
   ARCH = "unknown"
@@ -220,9 +220,9 @@ solution("xenia")
   uuid("931ef4b0-6170-4f7a-aaf2-0fece7632747")
   startproject("xenia-app")
   architecture("x86_64")
-  if os.is("linux") then
+  if os.istarget("linux") then
     platforms({"Linux"})
-  elseif os.is("windows") then
+  elseif os.istarget("windows") then
     platforms({"Windows"})
   end
   configurations({"Checked", "Debug", "Release"})
@@ -261,7 +261,7 @@ solution("xenia")
   include("src/xenia/ui/vulkan")
   include("src/xenia/vfs")
 
-  if os.is("windows") then
+  if os.istarget("windows") then
     include("src/xenia/apu/xaudio2")
     include("src/xenia/hid/winkey")
     include("src/xenia/hid/xinput")