Fix root_of_unity warning and doctests errors. When mpc version != 1.0.0

[vinklein]

Currently if gmpy2 is used with a mpc version older than 1.1.0 it generate a compilation warning and doctests failures as gmpy2.root_of_unity attribute is not defined.

Example of doctests failures :
Failed example:
    gmpy2.root_of_unity(1,2)
Exception raised:
    Traceback (most recent call last):
      File "/opt/python/3.5.6/lib/python3.5/doctest.py", line 1321, in __run
        compileflags, 1), test.globs)
      File "<doctest test_mpc.txt[114]>", line 1, in <module>
        gmpy2.root_of_unity(1,2)
    AttributeError: module 'gmpy2' has no attribute 'root_of_unity'

see: https://travis-ci.org/vinklein/gmpy/builds/509298994

• Fix gmpy2_mpc_misc.h to avoid generating warning
• Create a doctest flag and a doctest parser to SKIP doctests if root_of_unity is not defined
• Flag the appropriate doctests

[casevh]

Thanks!