setup.c: Prepare for Windows directory separators.

This turns two switch/case statements into an if-else-if cascade because
we later do not want to have

        case '/':
    #ifdef __MINGW32__
        case '\\':
    #endif

but use a predicate is_dir_sep(foo) in order to check for the directory
separator.
Signed-off-by: NJohannes Sixt <johannes.sixt@telecom.at>

setup.c

@@ -26,24 +26,21 @@ static int sanitary_path_copy(char *dst, const char *src)
 		 * (4) "../"          -- strip one, eat slash and continue.
 		 */
 		if (c == '.') {
-			switch (src[1]) {
-			case '\0':
+			if (!src[1]) {
 				/* (1) */
 				src++;
-				break;
-			case '/':
+			} else if (src[1] == '/') {
 				/* (2) */
 				src += 2;
 				while (*src == '/')
 					src++;
 				continue;
-			case '.':
-				switch (src[2]) {
-				case '\0':
+			} else if (src[1] == '.') {
+				if (!src[2]) {
 					/* (3) */
 					src += 2;
 					goto up_one;
-				case '/':
+				} else if (src[2] == '/') {
 					/* (4) */
 					src += 3;
 					while (*src == '/')