[PHP] Add support for PHP7.

PHP5's C extension API has changed substantially so you need to use
-php7 to specify you want PHP7 compatible wrappers.
Fixes https://github.com/swig/swig/issues/571

Examples/php5/simple/example.c

@@ -0,0 +1,23 @@
+/* File : example.c */
+#include <stdio.h>
+
+/* A global variable */
+double Foo = 3.0;
+
+void print_Foo() {
+   printf("In C, Foo = %f\n",Foo);
+}
+
+/* Compute the greatest common divisor of positive integers */
+int gcd(int x, int y) {
+  int g;
+  g = y;
+  while (x > 0) {
+    g = x;
+    x = y % x;
+    y = g;
+  }
+  return g;
+}
+
+