[premake] Remove redundant vectorextensions

Make clang version into a variable.

premake5.lua

@@ -126,7 +126,6 @@ filter({"configurations:Release", "platforms:Windows"}) -- "toolset:msc"
 filter("platforms:Linux")
   system("linux")
   toolset("clang")
-  vectorextensions("AVX")
   --buildoptions({
   --    "-mlzcnt",   -- (don't) Assume lzcnt is supported.
   --})
@@ -185,14 +184,15 @@ filter({"language:C++", "toolset:clang"}) -- "platforms:Linux"
   })
 CLANG_BIN = os.getenv("CC") or _OPTIONS["cc"] or "clang"
 if os.istarget("linux") and string.contains(CLANG_BIN, "clang") then
-  if tonumber(string.match(os.outputof(CLANG_BIN.." --version"), "version (%d%d)")) >= 20 then
+  CLANG_VER = tonumber(string.match(os.outputof(CLANG_BIN.." --version"), "version (%d%d)"))
+  if CLANG_VER >= 20 then
     filter({"language:C++", "toolset:clang"}) -- "platforms:Linux"
       disablewarnings({
         "deprecated-literal-operator",   -- Needed only for tabulate
         "nontrivial-memcall",
       })
   end
-  if tonumber(string.match(os.outputof(CLANG_BIN.." --version"), "version (%d%d)")) >= 21 then
+  if CLANG_VER >= 21 then
     filter({"language:C++", "toolset:clang"}) -- "platforms:Linux"
       disablewarnings({
         "character-conversion",          -- Needed for utfcpp third-party library