Fix building with luarocks

Building with `luarocks make` was broken in f3c8bf6 since
DMPACK_USE_AMALGAMATION is not defined by default. Change defines in
lmpack.c to only use system libmpack if DMPACK_USE_SYSTEM is defined.

Author: tarruda

Makefile

@@ -54,12 +54,11 @@ INCLUDES = $(LUA_INCLUDE)
 LIBS =
 
 ifeq ($(USE_SYSTEM_MPACK),no)
-CFLAGS += -DMPACK_USE_AMALGAMATION
 MPACK_SRC = mpack-src
 else
 MPACK_SRC =
 LIBS += $(shell $(PKG_CONFIG) --libs mpack 2>/dev/null || echo "-lmpack")
-CFLAGS += $(shell $(PKG_CONFIG) --cflags mpack 2> /dev/null)
+CFLAGS += -DMPACK_USE_SYSTEM $(shell $(PKG_CONFIG) --cflags mpack 2> /dev/null)
 endif
 
 LUA_CMOD_INSTALLDIR ?= $(shell $(PKG_CONFIG) --variable=INSTALL_CMOD $(LUA_IMPL) 2>/dev/null || echo "/usr/lib/lua/$(MPACK_LUA_VERSION_NOPATCH)")

lmpack.c

@@ -23,11 +23,11 @@
 #include <lua.h>
 #include <luaconf.h>
 
-#ifdef MPACK_USE_AMALGAMATION
+#ifdef MPACK_USE_SYSTEM
+# include <mpack.h>
+#else
 # define MPACK_API static
 # include "mpack-src/src/mpack.c"
-#else
-# include <mpack.h>
 #endif
 
 #define UNPACKER_META_NAME "mpack.Unpacker"