Fails to build with Clang

[RossComputerGuy]

directfb> ../../lib/direct/util.h:223:6: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
directfb>   223 |      register int ret = 0;
directfb>       |      ^~~~~~~~
directfb> ../../lib/direct/util.h:328:6: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
directfb>   328 |      register int ret = 0;
directfb>       |      ^~~~~~~~
directfb> In file included from String.cpp:37:
directfb> ./String.h:126:51: error: format argument not a string type
directfb>   126 |      PrintF( const _CharT *format, ... )          D_FORMAT_PRINTF(2);
directfb>       |              ~~~~~~~~~~~~~~~~~~~~                 ^               ~
directfb> ../../lib/direct/util.h:102:51: note: expanded from macro 'D_FORMAT_PRINTF'
directfb>   102 | #define D_FORMAT_PRINTF(n)         __attribute__((__format__ (__printf__, n, n+1)))
directfb>       |                                                   ^                       ~
directfb> In file included from String.cpp:37:
directfb> ./String.h:131:55: error: format argument not a string type
directfb>   131 |      static StringBase F( const _CharT *format, ... ) D_FORMAT_PRINTF(1);
directfb>       |                           ~~~~~~~~~~~~~~~~~~~~        ^               ~
directfb> ../../lib/direct/util.h:102:51: note: expanded from macro 'D_FORMAT_PRINTF'
directfb>   102 | #define D_FORMAT_PRINTF(n)         __attribute__((__format__ (__printf__, n, n+1)))
directfb>       |                                                   ^                       ~
directfb> String.cpp:111:19: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
directfb>   111 |      char     buf[stack_buffer];
directfb>       |                   ^~~~~~~~~~~~
directfb> String.cpp:111:19: note: function parameter 'stack_buffer' with unknown value cannot be used in a constant expression
directfb> String.cpp:108:58: note: declared here
directfb>   108 | String::PrintF( const char *format, va_list args, size_t stack_buffer )
directfb>       |                                                          ^
directfb> 3 warnings and 2 errors generated.
directfb> make[4]: *** [Makefile:918: String.lo] Error 1
directfb> make[4]: Leaving directory '/build/source/lib/direct'
directfb> make[3]: *** [Makefile:975: all-recursive] Error 1
directfb> make[3]: Leaving directory '/build/source/lib/direct'
directfb> make[2]: *** [Makefile:513: all-recursive] Error 1
directfb> make[2]: Leaving directory '/build/source/lib'
directfb> make[1]: *** [Makefile:704: all-recursive] Error 1
directfb> make[1]: Leaving directory '/build/source'
directfb> make: *** [Makefile:572: all] Error 2