Factor out Windows utility function get_system_dir().

The code to find out the location of the c:\windows\system32 directory
was already present, in load_system32_dll(). Now it's moved out into a
function of its own, so it can be called in other contexts.

windows/CMakeLists.txt

@@ -10,6 +10,7 @@ add_sources_from_current_dir(utils
   utils/filename.c
   utils/fontspec.c
   utils/getdlgitemtext_alloc.c
+  utils/get_system_dir.c
   utils/get_username.c
   utils/is_console_handle.c
   utils/load_system32_dll.c

windows/platform.h

@@ -566,6 +566,7 @@ HWND event_log_window(void);
 extern DWORD osMajorVersion, osMinorVersion, osPlatformId;
 void init_winver(void);
 void dll_hijacking_protection(void);
+const char *get_system_dir(void);
 HMODULE load_system32_dll(const char *libname);
 const char *win_strerror(int error);
 void restrict_process_acl(void);

windows/utils/get_system_dir.c

@@ -0,0 +1,21 @@
+/*
+ * Wrapper function around GetSystemDirectory that deals with
+ * allocating the output buffer, and also caches the result for future
+ * calls.
+ */
+
+#include "putty.h"
+
+const char *get_system_dir(void)
+{
+    static char *sysdir = NULL;
+    static size_t sysdirsize = 0;
+
+    if (!sysdir) {
+        size_t len;
+        while ((len = GetSystemDirectory(sysdir, sysdirsize)) >= sysdirsize)
+            sgrowarray(sysdir, sysdirsize, len);
+    }
+
+    return sysdir;
+}

windows/utils/load_system32_dll.c

@@ -8,18 +8,10 @@
 
 HMODULE load_system32_dll(const char *libname)
 {
-    static char *sysdir = NULL;
-    static size_t sysdirsize = 0;
     char *fullpath;
     HMODULE ret;
 
-    if (!sysdir) {
-        size_t len;
-        while ((len = GetSystemDirectory(sysdir, sysdirsize)) >= sysdirsize)
-            sgrowarray(sysdir, sysdirsize, len);
-    }
-
-    fullpath = dupcat(sysdir, "\\", libname);
+    fullpath = dupcat(get_system_dir(), "\\", libname);
     ret = LoadLibrary(fullpath);
     sfree(fullpath);
     return ret;