add libpd
- add a libpd folder cloned from libpd emscripten branch with only necessary folders and files
- update libpd/CMakeLists.txt, libpd/Makefile, and libpd/samples/emscripten/pdtest/Makefile
- remove PERTHREAD preprocessor variables
- replace STUFF->st_ with sys_
- remove undeclared function calls (sys_init_fdpoll()/pd_globallock()/pd_globalunlock())
- remove unsupported function declarations and definitions (libpd_getdollarzero(), PDINSTANCE related functions, libpd_midibyte()/libpd_sysex()/libpd_sysrealtime())
- change libpd_openfile()'s type to void
- update dummy routines needed
- set sys_nogui to 1 before calling pd_init()