Support pthreads on PLATFORM_WEB compilation

src/Makefile

@@ -322,7 +322,7 @@ ifeq ($(PLATFORM),PLATFORM_WEB)
     # --profiling                # include information for code profiling
     # --memory-init-file 0       # to avoid an external memory initialization code file (.mem)
     # --preload-file resources   # specify a resources folder for data compilation
-    CFLAGS += -s USE_GLFW=3
+    CFLAGS += -s USE_GLFW=3 -s USE_PTHREADS=1
 endif
 ifeq ($(PLATFORM),PLATFORM_ANDROID)
     # Compiler flags for arquitecture