Allow building without libxslt debugger support

[mohd-akram]

Debugger support is disabled by default since libxslt 1.1.43. Perhaps the debugger code can be wrapped in #ifdef WITH_DEBUGGER.

[rosswmerrifield]

Yes, it fails to build for me because of this

xmlsoft.h:76:5: error: implicit declaration of function 'xsltSetDebuggerCallbacks'; did you mean 'xsltSetDebuggerCallbacksHelper'? [-Wimplicit-function-declaration] 76 | xsltSetDebuggerCallbacks(NUM_ARRAY(cb), &cb); | ^~~~~~~~~~~~~~~~~~~~~~~~ | xsltSetDebuggerCallbacksHelper