graphics/libfreehand: fix build with clang 16

Since clang 16 (and gcc 11) the default C++ standard is now gnu++17.
Because graphics/libfreehand's configure scripts and Makefiles do not
explicitly set the C++ standard, this leads to several errors:

  In file included from FreeHandDocument.cpp:14:
  In file included from ./FHParser.h:15:
  /usr/local/include/lcms2.h:1291:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  typedef cmsInt32Number (* cmsSAMPLER16)   (CMSREGISTER const cmsUInt16Number In[],
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^
  /usr/local/include/lcms2.h:1292:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
                                             CMSREGISTER cmsUInt16Number Out[],
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^
  /usr/local/include/lcms2.h:1293:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
                                             CMSREGISTER void * Cargo);
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^
  /usr/local/include/lcms2.h:1295:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
  typedef cmsInt32Number (* cmsSAMPLERFLOAT)(CMSREGISTER const cmsFloat32Number In[],
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^
  /usr/local/include/lcms2.h:1296:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
                                             CMSREGISTER cmsFloat32Number Out[],
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^
  /usr/local/include/lcms2.h:1297:44: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
                                             CMSREGISTER void * Cargo);
                                             ^~~~~~~~~~~~
  /usr/local/include/lcms2.h:158:23: note: expanded from macro 'CMSREGISTER'
  #  define CMSREGISTER register
                        ^

Add -D CMS_NO_REGISTER_KEYWORD to CPPFLAGS, to avoid lcms2.h using
register keywords in its declarations.

PR:		271421
Approved by:	sunpoet (maintainer)
MFH:		2023Q2