Fix detecting the system path

The normalize function don't do well it's job.
Given a path like that,  the regexp "/[^/]+/.."  will match 3 times
 /usr/lib/gcc/x86_64-unknown-linux-gnu/4.8.2/../../../../include/c++/4.8.2
                                      [       ][    ][         ]
The second match is wrong as it will remove /../..

Use sub instead of gsub which only remove one match at the time.

Change-Id: I0657bc603e521c9e53b9f50d2481dce184b64bad
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

configure

@@ -3508,7 +3508,7 @@ function normalize(dir)
 {
     do {
         odir = dir
-        gsub(/\/[^\/]+\/\.\./, "", dir)
+        sub(/\/[^\/]+\/\.\./, "", dir)
     } while (dir != odir);
     do {
         odir = dir