[build] Rework how the target architecture is looked up at build time.

We now check the system Mono architecture in non-Windows platforms, this is needed since Mono 5 defaults to amd64.

build/Helpers.lua

@@ -2,7 +2,6 @@
 
 newoption {
    trigger     = "arch",
-   default       = "x86",
    description = "Choose a particular architecture / bitness",
    allowed = {
       { "x86",  "x86 32-bits" },
@@ -10,6 +9,27 @@ newoption {
    }
 }
 
+explicit_target_architecture = _OPTIONS["arch"]
+
+function is_64_bits_mono_runtime()
+  result, errorcode = os.outputof("mono --version")
+  local arch = string.match(result, "Architecture:%s*([%w]+)")
+  return arch == "amd64"
+end
+
+function target_architecture()
+  -- Default to 32-bit on Windows and Mono architecture otherwise.
+  if explicit_target_architecture ~= nil then
+    return explicit_target_architecture
+  end
+  if os.is("windows") then return "x86" end
+  return is_64_bits_mono_runtime() and "x64" or "x86"
+end
+
+if not _OPTIONS["arch"] then
+  _OPTIONS["arch"] = target_architecture()
+end
+
 action = _ACTION or ""
 
 basedir = path.getdirectory(_PREMAKE_COMMAND)

build/Tests.lua

@@ -73,7 +73,7 @@ function SetupTestGeneratorProject(name, depends)
 end
 
 function SetupTestGeneratorBuildEvent(name)
-  local monoExe = _OPTIONS["arch"] == "x64" and "mono64" or "mono"
+  local monoExe = target_architecture() == "x64" and "mono64" or "mono"
   local runtimeExe = os.is("windows") and "" or monoExe .. " --debug "
   if string.starts(action, "vs") then
     local exePath = SafePath("$(TargetDir)" .. name .. ".Gen.exe")

build/scripts/LLVM.lua

@@ -64,7 +64,7 @@ end
 
 function get_toolset_configuration_name(arch)
   if not arch then
-    arch = _OPTIONS["arch"]
+    arch = target_architecture()
   end
 
   if os.is("windows") then
@@ -276,7 +276,7 @@ function build_llvm(llvm_build)
 	else
 		local options = os.is("macosx") and
 			"-DLLVM_ENABLE_LIBCXX=true" or ""
-		local is32bits = _OPTIONS["arch"] == "x86"
+		local is32bits = target_architecture() == "x86"
 		if is32bits then
 			options = options .. is32bits and " -DLLVM_BUILD_32_BITS=true" or ""
 		end