Fix building with GCC-6

[Peter-Levine]

Since C++11, there has existed std::error_category::equivalent. OpenNI defines a macro equivalent. During header inclusion, a GCC header calls the function equivalent but that symbol is hidden by the name clash with the macro equivalent leading to:

In file included from ../../../../../Samples/NiViewer/glh/glh_obs.h:62:0,
                 from ../../../../../Samples/NiViewer/NiViewer.cpp:62:
/usr/lib/gcc/x86_64-pc-linux-gnu/6.2.0/include/g++-v6/system_error:107:5: error: expected unqualified-id before ‘int’
     equivalent(int __i, const error_condition& __cond) const noexcept;
     ^

This is easily remedied by changing all instances of macro equivalent to is_equivalent.

[rguillome]

This correction suits well for this error too :
../../../../../Samples/NiViewer/glh/glh_linear.h:80:63: error: no match for ‘operator>’ (operand types are ‘const std::error_code’ and ‘float’) #define equivalent(a,b) (((a < b + GLH_EPSILON) && (a > b - GLH_EPSILON)) ? true : false)
and it's the same as the PR #101