Tests for cpdef extern func wrapping.

tests/run/cpdef_extern_func.pxd

+cdef extern from "math.h":
+    cpdef double pxd_sqrt "sqrt"(double)

tests/run/cpdef_extern_func.pyx

+__doc__ = """
+>>> sqrt(1)
+1.0
+>>> pyx_sqrt(4)
+2.0
+>>> pxd_sqrt(9)
+3.0
+>>> log(10)
+Traceback (most recent call last):
+NameError: name 'log' is not defined
+"""
+
+cdef extern from "math.h":
+    cpdef double sqrt(double)
+    cpdef double pyx_sqrt "sqrt"(double)
+    cdef double log(double) # not wrapped