Doc cmake: Mention workaround add_definitions(-D_USE_MATH_DEFINES).
Doc cmake: Mention workaround add_definitions(-D_USE_MATH_DEFINES).

This will save user lives :) .

Change-Id: I0db63be9a75a4b6947bd78a94757953f9b71997d

aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi,urbiforge/urbi