hpp-fcl v1.7.3 doesn't compile on the standard default configuration on
ArchLinux (GCC 11.1.0, Eigen 3.3.9, Boost 1.75.0) and on
Fedora 34 (GCC 11.1.1, Eigen 3.3.9, Boost 1.75.0), with the following
error message:

```
In file included from /usr/include/eigen3/Eigen/Core:368,
from ./hpp-fcl-1.7.3/include/hpp/fcl/data_types.h:41,
from ./hpp-fcl-1.7.3/include/hpp/fcl/collision.h:43,
from ./hpp-fcl-1.7.3/test/serialization.cpp:40:
/usr/include/eigen3/Eigen/src/Core/util/ForwardDeclarations.h: In instantiation of 'struct Eigen::internal::accessors_level':
/usr/include/eigen3/Eigen/src/Core/util/ForwardDeclarations.h:109:58:   required by substitution of 'template class SPT> void boost::serialization::load(Archive&, SPT&, unsigned int) [with Archive = boost::archive::text_iarchive; SPT = ]'
/usr/include/boost/serialization/split_free.hpp:58:13:   required from 'static void boost::serialization::free_loader::invoke(Archive&, T&, unsigned int) [with Archive = boost::archive::text_iarchive; T = Eigen::Map, 0, Eigen::Stride<0, 0> >]'
/usr/include/boost/serialization/split_free.hpp:74:18:   required from 'void boost::serialization::split_free(Archive&, T&, unsigned int) [with Archive = boost::archive::text_iarchive; T = Eigen::Map, 0, Eigen::Stride<0, 0> >]'
./hpp-fcl-1.7.3/include/hpp/fcl/serialization/eigen.h:82:17:   required from 'void boost::serialization::serialize(Archive&, Eigen::Map&, unsigned int) [with Archive = boost::archive::text_iarchive; PlainObjectBase = Eigen::Matrix; int MapOptions = 0; StrideType = Eigen::Stride<0, 0>]'
/usr/include/boost/serialization/serialization.hpp:109:14:   required from 'void boost::serialization::serialize_adl(Archive&, T&, unsigned int) [with Archive = boost::archive::text_iarchive; T = Eigen::Map, 0, Eigen::Stride<0, 0> >]'
/usr/include/boost/archive/detail/iserializer.hpp:187:40:   [ skipping 29 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/include/boost/archive/detail/common_iarchive.hpp:67:22:   required from 'void boost::archive::detail::common_iarchive::load_override(T&) [with T = hpp::fcl::BVHModelBase; Archive = boost::archive::text_iarchive]'
/usr/include/boost/archive/basic_text_iarchive.hpp:70:52:   required from 'void boost::archive::basic_text_iarchive::load_override(T&) [with T = hpp::fcl::BVHModelBase; Archive = boost::archive::text_iarchive]'
/usr/include/boost/archive/text_iarchive.hpp:82:52:   required from 'void boost::archive::text_iarchive_impl::load_override(T&) [with T = hpp::fcl::BVHModelBase; Archive = boost::archive::text_iarchive]'
/usr/include/boost/archive/detail/interface_iarchive.hpp:68:36:   required from 'Archive& boost::archive::detail::interface_iarchive::operator>>(T&) [with T = hpp::fcl::BVHModelBase; Archive = boost::archive::text_iarchive]'
./hpp-fcl-1.7.3/test/serialization.cpp:122:10:   required from 'void test_serialization(const T&, T&, int) [with T = hpp::fcl::BVHModelBase]'
./hpp-fcl-1.7.3/test/serialization.cpp:231:23:   required from here
/usr/include/eigen3/Eigen/src/Core/util/ForwardDeclarations.h:32:48: error: incomplete type 'Eigen::internal::traits' used in nested name specifier
32 |   enum { has_direct_access = (traits::Flags & DirectAccessBit) ? 1 : 0,
|                                                ^~~~~
/usr/include/eigen3/Eigen/src/Core/util/ForwardDeclarations.h:33:47: error: incomplete type 'Eigen::internal::traits' used in nested name specifier
33 |          has_write_access = (traits::Flags & LvalueBit) ? 1 : 0,
|                                               ^~~~~
```

This bug was already reported at https://stackoverflow.com/questions/54534047/eigen-matrix-boostserialization-c17
and discussed at https://gitlab.com/libeigen/eigen/-/issues/1676

A minimal reproducible example is provided at https://godbolt.org/z/uIy1Uu
This example shows that this bug is triggered only on GCC >= 7 (other
compilers are not affected) and on C++17 (which is the default for
GCC >= 11).

Also, hpp-fcl 1.7.3 compilation is fine on Arch either by using clang or
by explicitely setting GCC on C++14.

This commit implements the workaround provided by Christoph Hertzberg on
the Eigen issue discussion.

Another workaround is already available in Eigen >= 3.3.8:
https://gitlab.com/libeigen/eigen/-/commit/2aa9eb3ce8fa6b2d61dce5be9d6d6460a28080c4
But this doesn't fix the build of hpp-fcl 1.7.3.

Another workaround was proposed to boost::serialization, but was
rejected: https://github.com/boostorg/serialization/pull/144

The main bug in GCC got no attention for the last 2 years:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84075