Move the sqlite3_temp_directory global variable out of the OS porting layer

and into main.c so that it is more accessible to non-standard OS porting
layers. (CVS 3759)

FossilOrigin-Name: 23653f7aab6609f69948a7366bef9ebd0fd4d0b1

src/os_win.c

@@ -898,13 +898,6 @@ static int winOpenDirectory(
   return SQLITE_OK;
 }
 
-/*
-** If the following global variable points to a string which is the
-** name of a directory, then that directory will be used to store
-** temporary files.
-*/
-char *sqlite3_temp_directory = 0;
-
 /*
 ** Create a temporary file name in zBuf.  zBuf must be big enough to
 ** hold at least SQLITE_TEMPNAME_SIZE characters.