Make os.tempfolder work correctly for MinGW & MSVC (#7443)

builtin/mainmenu/common.lua

@@ -171,6 +171,23 @@ os.tempfolder = function()
 	local filetocheck = os.tmpname()
 	os.remove(filetocheck)
 
+	-- https://blogs.msdn.microsoft.com/vcblog/2014/06/18/c-runtime-crt-features-fixes-and-breaking-changes-in-visual-studio-14-ctp1/
+	--   The C runtime (CRT) function called by os.tmpname is tmpnam.
+	--   Microsofts tmpnam implementation in older CRT / MSVC releases is defective.
+	--   tmpnam return values starting with a backslash characterize this behavior.
+	-- https://sourceforge.net/p/mingw-w64/bugs/555/
+	--   MinGW tmpnam implementation is forwarded to the CRT directly.
+	-- https://sourceforge.net/p/mingw-w64/discussion/723797/thread/55520785/
+	--   MinGW links to an older CRT release (msvcrt.dll).
+	--   Due to legal concerns MinGW will never use a newer CRT.
+	--
+	--   Make use of TEMP to compose the temporary filename if an old
+	--   style tmpnam return value is detected.
+	if filetocheck:sub(1, 1) == "\\" then
+		local tempfolder = os.getenv("TEMP")
+		return tempfolder .. filetocheck
+	end
+
 	local randname = "MTTempModFolder_" .. math.random(0,10000)
 	local backstring = filetocheck:reverse()
 	return filetocheck:sub(0, filetocheck:len() - backstring:find(DIR_DELIM) + 1) ..