Fix PMP on i386/i686 (32 bits, with the x87 FPU)

[lrineau]

Summary of Changes

Fix PMP on i386/i686 (32 bits, with the x87 FPU).

Release Management

• Affected package(s): PMP, Number_types

[sloriot]

PMP almost fully red in CGAL-6.1-Ic-164

[MaelRL]

Simply:

    FT lower_bound = square(FT(error_bound));
    CGAL_precondition(sq_initial_bound >= lower_bound);

?

[sloriot]

Does not fix issues in CGAL-6.1-Ic-168

[sloriot]

Successfully tested in CGAL-6.2-Ic-43