system: add PATH_SEPARATOR

I am aware of the std::filesystem::path::preferred_separator, but we
are targeting C++14 and we need this as a precompiler macro to allow
concatenation of the path strings in precompile stage.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/stdex/system.hpp

@@ -31,6 +31,14 @@
 #endif
 #endif
 
+#if defined(_WIN32)
+#define PATH_SEPARATOR '\\'
+#define PATH_SEPARATOR_STR "\\"
+#else
+#define PATH_SEPARATOR '/'
+#define PATH_SEPARATOR_STR "/"
+#endif
+
 namespace stdex
 {
 	///