Add thread compile option

Author: ethanaobrien

Makefile.libretro

@@ -1,5 +1,6 @@
 # Makefile for PCSX ReARMed (libretro)
 
+EMULATORJS_THREADS ?= 0
 DEBUG ?= 0
 WANT_ZLIB ?= 1
 HAVE_CHD ?= 1
@@ -541,7 +542,11 @@ else ifeq ($(platform), emscripten)
  fpic := -fPIC
  CFLAGS += -DNO_DYLIB -DNO_SOCKET
  CFLAGS += -msimd128 -ftree-vectorize
- LIBPTHREAD :=
+ ifeq ($(EMULATORJS_THREADS), 1)
+ THREAD_RENDERING = 1
+ else
+ LIBPTHREAD :=
+ endif
  LIBDL :=
  DYNAREC := 0
  STATIC_LINKING = 1