Fix macosx build error related to missing libxml2

This commit fixes the following build error:

[ 19%] Built target i2d
[ 19%] Built target dcm2xml
[ 19%] Built target dcmdump
[ 19%] Built target dcmftest
make[5]: *** No rule to make target `/Users/pieper/ctk/latest/dcmjs-build/LibXml2/.libs/libxml2.so', needed by `lib/libxml2dcm.so.3.6.1'.  Stop.
make[4]: *** [dcmdata/apps/CMakeFiles/xml2dcm.dir/all] Error 2
make[3]: *** [all] Error 2
make[2]: *** [DCMTK-prefix/src/DCMTK-stamp/DCMTK-build] Error 2
make[1]: *** [CMakeFiles/DCMTK.dir/all] Error 2
make: *** [all] Error 2