Provide support for the complex library (#2891)

Cython/Includes/libc/complex.pxd

+cdef extern from "<complex.h>" nogil:
+    # Trigonometric functions.
+    double complex cacos(double complex z)
+    double complex casin(double complex z)
+    double complex catan(double complex z)
+    double complex ccos(double complex z)
+    double complex csin(double complex z)
+    double complex ctan(double complex z)
+
+    # Hyperbolic functions.
+    double complex cacosh(double complex z)
+    double complex casinh(double complex z)
+    double complex catanh(double complex z)
+    double complex ccosh(double complex z)
+    double complex csinh(double complex z)
+    double complex ctanh(double complex z)
+
+    # Exponential and logarithmic functions.
+    double complex cexp(double complex z)
+    double complex clog(double complex z)
+    double complex clog10(double complex z)
+
+    # Power functions.
+    double complex cpow(double complex x, double complex y)
+    double complex csqrt(double complex z)
+
+    # Absolute value, conjugates, and projection.
+    double cabs(double complex z)
+    double carg(double complex z)
+    double complex conj(double complex z)
+    double complex cproj(double complex z)
+
+    # Decomposing complex values.
+    double cimag(double complex z)
+    double creal(double complex z)

tests/run/complex_numbers_cmath_T2891.pyx

+# ticket: 2891
+# tag: c, no-cpp
+
+cdef extern from "complex_numbers_c99_T398.h": pass
+
+from libc.complex cimport cimag, creal, cabs, carg
+
+
+def test_decomposing(double complex z):
+    """
+    >>> test_decomposing(3+4j)
+    (3.0, 4.0, 5.0, 0.9272952180016122)
+    """
+
+    return (creal(z), cimag(z), cabs(z), carg(z))