libxml2 needs upstream patch to compile with musl libc
https://gitlab.gnome.org/GNOME/libxml2/commit/fff8a6b87e05200a0ad0af6f86c2e859c7de9172
patch above solves this build error:
[ 9%] For vtkIOXMLParser - updating vtkIOXMLParserHierarchy.txt
[ 9%] Built target vtkImagingStatisticsHierarchy
/opt/build/VTKv9/ThirdParty/libxml2/vtklibxml2/threads.c:76:27: error: macro "pthread_equal" requires 2 arguments, but only 1 given
extern int pthread_equal ()
^
/opt/build/VTKv9/ThirdParty/libxml2/vtklibxml2/threads.c:76:12: error: 'pthread_equal' redeclared as different kind of symbol
extern int pthread_equal ()
^~~~~~~~~~~~~
In file included from /opt/build/VTKv9/ThirdParty/libxml2/vtklibxml2/threads.c:28:0:
/usr/include/pthread.h:84:5: note: previous declaration of 'pthread_equal' was here
int pthread_equal(pthread_t, pthread_t);
^~~~~~~~~~~~~