Makefile: Add MT option to enable the 'preview_mt' flag (#4993)

This PR add an MT option to the Makefile. When make is invoked with 'MT=1',
the 'preview_mt' flag is passed to the Crystal compiler.

It doesn't mean that invidious fully supports multi-threading, but at least
it provides an easy way for trying that out.

No related issue.

Makefile

@@ -7,6 +7,11 @@ STATIC   := 0
 
 NO_DBG_SYMBOLS := 0
 
+# Enable multi-threading.
+# Warning: Experimental feature!!
+# invidious is not stable when MT is enabled.
+MT := 0
+
 
 FLAGS ?=
 
@@ -19,6 +24,10 @@ ifeq ($(STATIC), 1)
   FLAGS += --static
 endif
 
+ifeq ($(MT), 1)
+  FLAGS += -Dpreview_mt
+endif
+
 
 ifeq ($(NO_DBG_SYMBOLS), 1)
   FLAGS += --no-debug