use the latest released version of libxml2 (v2.13.1)

[paulhiggs]

Small changes needed to the various xml.js build scripts

• libxml2 places some macro definition files in the libxml2/m4 directory, so just delete any files placed there by local buid process
• latest version of emcc (used 3.1.61) does not support the --memory-init-file 0 option
• configure libxml2 to build withing using dynamic modules
• link static file into build