work around missing relative cimport support in cythonize()

Cython/Includes/libc/locale.pxd

 # 7.11 Localization <locale.h>
 
 # deprecated cimport for backwards compatibility:
-from .string cimport const_char
+from libc.string cimport const_char
 
 
 cdef extern from "locale.h" nogil:

Cython/Includes/libc/stdio.pxd

@@ -2,7 +2,7 @@
 
 
 # deprecated cimports for backwards compatibility:
-from .string cimport const_char, const_void
+from libc.string cimport const_char, const_void
 
 
 cdef extern from "stdio.h" nogil:

Cython/Includes/libc/stdlib.pxd

 # 7.20 General utilities <stdlib.h>
 
 # deprecated cimports for backwards compatibility:
-from .string cimport const_char, const_void
+from libc.string cimport const_char, const_void
 
 
 cdef extern from "stdlib.h" nogil: