Change the pkg-config command to properly detect the Lua library also on Debian/Ubuntu.

externals/Makefile

@@ -1406,8 +1406,12 @@ pddp_clean:
 
 #------------------------------------------------------------------------------#
 # PDLUA
+
+LUA_CFLAGS = $(shell (pkg-config --cflags lua5.3 || pkg-config --cflags lua5.2 || pkg-config --cflags lua) 2> /dev/null)
+LUA_LIBS = $(shell (pkg-config --libs lua5.3 || pkg-config --libs lua5.2 || pkg-config --libs lua) 2> /dev/null)
+
 pdlua:
-	make -C $(externals_src)/pd-lua LUA_CFLAGS="$(shell pkg-config --cflags lua)" LUA_LIBS="$(shell pkg-config --libs lua)" PD_PATH=$(pd_src) CFLAGS="$(CFLAGS)"
+	make -C $(externals_src)/pd-lua LUA_CFLAGS="$(LUA_CFLAGS)" LUA_LIBS="$(LUA_LIBS)" PD_PATH=$(pd_src) CFLAGS="$(CFLAGS)"
 
 pdlua_install:
 	make -C $(externals_src)/pd-lua DESTDIR="$(DESTDIR)" objectsdir="$(objectsdir)" install