dl: add a complete tested shared library generation/usage example

vlib/dl/dl_nix.c.v

@@ -1,11 +1,10 @@
 module dl
 
 #include <dlfcn.h>
-
 pub const (
-	rtld_now = C.RTLD_NOW
+	rtld_now  = C.RTLD_NOW
 	rtld_lazy = C.RTLD_LAZY
-	dl_ext = '.so'
+	dl_ext    = get_shared_library_extension()
 )
 
 fn C.dlopen(filename charptr, flags int) voidptr
@@ -28,3 +27,14 @@ pub fn close(handle voidptr) bool {
 pub fn sym(handle voidptr, symbol string) voidptr {
 	return C.dlsym(handle, symbol.str)
 }
+
+pub fn get_shared_library_extension() string {
+	mut res := '.so'
+	$if macos {
+		res = '.dylib'
+	}
+	$if windows {
+		res = '.dll'
+	}
+	return res
+}