Error with old Boost versions and fenv

[bchretien]

See this Travis log:

In file included from /home/travis/build/roboptim/roboptim-trajectory/tests/b-spline.cc:21:
In file included from /home/travis/build/roboptim/roboptim-trajectory/include/roboptim/trajectory/cubic-b-spline.hh:27:
In file included from /home/travis/build/roboptim/roboptim-trajectory/include/roboptim/trajectory/polynomial-3.hh:22:
In file included from /home/travis/build/roboptim/roboptim-trajectory/include/roboptim/trajectory/polynomial.hh:337:
In file included from /home/travis/build/roboptim/roboptim-trajectory/include/roboptim/trajectory/polynomial.hxx:28:
In file included from /usr/include/boost/math/special_functions/factorials.hpp:13:
In file included from /usr/include/boost/math/special_functions/gamma.hpp:27:
In file included from /usr/include/boost/math/tools/series.hpp:16:
In file included from /usr/include/boost/math/tools/config.hpp:260:
In file included from /usr/include/boost/detail/fenv.hpp:25:
/usr/include/x86_64-linux-gnu/bits/fenv.h:30:5: error: redefinition of enumerator 'FE_INVALID'
    FE_INVALID = 0x01,
    ^
/usr/include/x86_64-linux-gnu/bits/fenv.h:31:20: note: expanded from macro 'FE_INVALID'
#define FE_INVALID      FE_INVALID
                        ^
/usr/include/x86_64-linux-gnu/bits/fenv.h:30:5: note: previous definition is here
    FE_INVALID = 0x01,
    ^

The Boost version is 1.46.1 (March 12th, 2011...).