Typo in feature guard macros

[nicola-gigante]

There's a typo in the header's feature macros. HIGHER is spelled "HIGHTER".
At first I was worried of the typo leading to some bugs, but it seems to be coherently misspelled in the whole header so it's ok.

Nevertheless, I felt like reporting it would be useful anyway...

On line 45:

# if defined __clang_major__
#   if (__clang_major__ == 3 && __clang_minor__ >= 5)
#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#   elif (__clang_major__ > 3)
#     define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#   endif
#   if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_
#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#   elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2)
#     define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_
#   endif
# endif

On line 87:

# if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L)