Modernize "C" includes in biconcor.

This is one of those little chores in managing a long-lived C++
project: standard C headers like stdio.h and math.h now have their own
place in the C++ standard as resp. cstdio, cmath, and so on.  In this
branch the #include names are updated for the biconcor/ subdirectory.

C++11 adds cstdint, but to support compilation with the previous
standard, that change is left for later.

biconcor/Alignment.cpp

@@ -2,7 +2,7 @@
 
 #include <fstream>
 #include <string>
-#include <stdlib.h>
+#include <cstdlib>
 #include <cstring>
 
 namespace

biconcor/Mismatch.cpp

@@ -4,7 +4,7 @@
 #include <iostream>
 #include <cstring>
 #include <string>
-#include <stdlib.h>
+#include <cstdlib>
 
 #include "SuffixArray.h"
 #include "TargetCorpus.h"

biconcor/PhrasePairCollection.cpp

@@ -1,6 +1,6 @@
 #include "PhrasePairCollection.h"
 
-#include <stdlib.h>
+#include <cstdlib>
 #include <cstring>
 #include <algorithm>
 

biconcor/SuffixArray.cpp

@@ -2,7 +2,7 @@
 
 #include <fstream>
 #include <string>
-#include <stdlib.h>
+#include <cstdlib>
 #include <cstring>
 
 namespace

biconcor/TargetCorpus.cpp

@@ -2,7 +2,7 @@
 
 #include <fstream>
 #include <string>
-#include <stdlib.h>
+#include <cstdlib>
 #include <cstring>
 
 namespace