OS X build issues

[springmeyer]

tests

1. endian.h not available on OS X

In file included from t/geom/test_geos_wkb.cpp:5:
../include/osmium/geom/wkb.hpp:44:11: fatal error: 'endian.h' file not found
# include <endian.h>

2. -Werror,-Wsign-compare

In file included from t/basic/test_node.cpp:1:
include/catch.hpp:831:34: error: comparison of integers of different signs: 'int' and 'unsigned long long' [-Werror,-Wsign-compare]
            return opCast( lhs ) ==  opCast( rhs );
                   ~~~~~~~~~~~~~ ^   ~~~~~~~~~~~~~
include/catch.hpp:876:39: note: in instantiation of member function 'Catch::Internal::Evaluator<int, unsigned long long, 0>::evaluate' requested here
        return Evaluator<T1, T2, Op>::evaluate( lhs, rhs );
                                      ^
include/catch.hpp:1293:39: note: in instantiation of function template specialization 'Catch::Internal::compare<0, int, unsigned long long>' requested here
            .setResultType( Internal::compare<Op>( m_lhs, rhs ) )
                                      ^
include/catch.hpp:1236:16: note: in instantiation of function template specialization 'Catch::ExpressionLhs<const int &>::captureExpression<0, unsigned long long>' requested here
        return captureExpression<Internal::IsEqualTo>( rhs );
               ^
t/basic/test_node.cpp:47:15: note: in instantiation of function template specialization 'Catch::ExpressionLhs<const int &>::operator==<unsigned long long>' requested here
    REQUIRE(0 == node.positive_id());
              ^

examples

1. -Werror,-Wunused-variable

In file included from osmium_convert.cpp:14:
In file included from ../include/osmium/io/any_output.hpp:38:
In file included from ../include/osmium/io/opl_output.hpp:37:
../include/osmium/io/detail/opl_output_format.hpp:109:25: error: unused variable 'len' [-Werror,-Wunused-variable]
                    int len = snprintf(m_tmp_buffer, tmp_buffer_size, format, std::forward<TArgs>(args)...);
                        ^
In file included from osmium_convert.cpp:14:
In file included from ../include/osmium/io/any_output.hpp:40:
In file included from ../include/osmium/io/xml_output.hpp:37:
../include/osmium/io/detail/xml_output_format.hpp:100:25: error: unused variable 'len' [-Werror,-Wunused-variable]
                    int len = snprintf(buffer, max_size, format, value);
                        ^
2 errors generated.
make: *** [osmium_convert] Error 1

[joto]

Fixed by 5b70eb5, daece40, and ca575d4.