bpo-33182: Fix pointer types in _testembed (GH-6310) (GH-6311)

(cherry picked from commit 69f5c733)
Co-authored-by: Nick Coghlan <ncoghlan@gmail.com>

Misc/NEWS.d/next/Build/2018-03-30-14-55-48.bpo-33182.CePczb.rst

+The embedding tests can once again be built with clang 6.0

Programs/_testembed.c

@@ -166,16 +166,18 @@ static int test_pre_initialization_api(void)
 /* bpo-33042: Ensure embedding apps can predefine sys module options */
 static int test_pre_initialization_sys_options(void)
 {
-    /* We allocate a couple of the option dynamically, and then delete
+    /* We allocate a couple of the options dynamically, and then delete
      * them before calling Py_Initialize. This ensures the interpreter isn't
      * relying on the caller to keep the passed in strings alive.
      */
-    wchar_t *static_warnoption = L"once";
-    wchar_t *static_xoption = L"also_not_an_option=2";
+    const wchar_t *static_warnoption = L"once";
+    const wchar_t *static_xoption = L"also_not_an_option=2";
     size_t warnoption_len = wcslen(static_warnoption);
     size_t xoption_len = wcslen(static_xoption);
-    wchar_t *dynamic_once_warnoption = calloc(warnoption_len+1, sizeof(wchar_t));
-    wchar_t *dynamic_xoption = calloc(xoption_len+1, sizeof(wchar_t));
+    wchar_t *dynamic_once_warnoption = \
+             (wchar_t *) calloc(warnoption_len+1, sizeof(wchar_t));
+    wchar_t *dynamic_xoption = \
+             (wchar_t *) calloc(xoption_len+1, sizeof(wchar_t));
     wcsncpy(dynamic_once_warnoption, static_warnoption, warnoption_len+1);
     wcsncpy(dynamic_xoption, static_xoption, xoption_len+1);