Emscripten without shared memory must not use -pthread: Meson's threads
dependency injects it into compile/link args and the generated
harfbuzz.pc (Libs/Cflags), forcing every pkg-config consumer into
wasm shared-memory mode while dependencies remain single-threaded.
Skip the threads dependency and build with HB_NO_MT instead.

--- a/meson.build
+++ b/meson.build
@@ -348,7 +348,10 @@
 
 # threads
 thread_dep = null_dep
-if host_machine.system() != 'windows'
+if host_machine.system() == 'emscripten'
+  # No -pthread (it would poison harfbuzz.pc); no internal locking either.
+  add_project_arguments('-DHB_NO_MT', language: 'cpp')
+elif host_machine.system() != 'windows'
   thread_dep = dependency('threads', required: false)
 
   if thread_dep.found()