build: add optional USE_EXTERNAL_LIBDEFLATE

When setting USE_EXTERNAL_LIBDEFLATE=1 as a make(1) argument a system
version will be used and otherwise it will use the vendored libdeflate.

Makefile

@@ -1,5 +1,7 @@
 SRCDIR := $(abspath $(patsubst %/,%,$(dir $(abspath $(lastword $(MAKEFILE_LIST))))))
 
+USE_EXTERNAL_LIBDEFLATE ?= 0
+
 PREFIX ?= /usr/local
 BINDIR ?= $(PREFIX)/bin
 MANDIR ?= $(PREFIX)/share/man
@@ -50,16 +52,26 @@ ZOPFLI_C_SRC := $(ZOPFLI_C_DIR)/blocksplitter.c $(ZOPFLI_C_DIR)/cache.c \
 ZOPFLI_C_TMP := $(ZOPFLI_C_SRC:.c=.o)
 ZOPFLI_C_OBJ := $(patsubst $(SRCDIR)/%,$(OBJDIR)/%,$(ZOPFLI_C_TMP))
 
-EXTRA_LIBS =
 ifeq ($(OS),Windows_NT)
-	LIBDEFLATE=libdeflatestatic.lib
-	EXTRA_LIBS += -luuid
+	LIBDEFLATE := libdeflatestatic.lib
+	LIBS += -luuid
 else
-	LIBDEFLATE=libdeflate.a
+	LIBDEFLATE := libdeflate.a
 endif
 
-SRC_7ZIP = $(OBJDIR)/7zip/7zip.a
-SRC_LIBDEFLATE = $(SRCDIR)/libdeflate/$(LIBDEFLATE)
+SRC_7ZIP := $(OBJDIR)/7zip/7zip.a
+SRC_LIBDEFLATE := $(SRCDIR)/libdeflate/$(LIBDEFLATE)
+
+OBJS := $(SRC_CXX_OBJ) $(CLI_CXX_OBJ) $(ZOPFLI_C_OBJ) $(SRC_7ZIP)
+
+ifeq ($(USE_EXTERNAL_LIBDEFLATE), 0)
+	OBJS += $(SRC_LIBDEFLATE)
+else
+	CFLAGS_LIBDEFLATE = $(shell $(PKG_CONFIG) --cflags libdeflate)
+	LIBS_LIBDEFLATE = $(shell $(PKG_CONFIG) --libs libdeflate)
+	SRC_CXXFLAGS += $(CFLAGS_LIBDEFLATE)
+	LIBS += $(LIBS_LIBDEFLATE)
+endif
 
 .PHONY: all clean install uninstall
 
@@ -71,9 +83,8 @@ $(OBJDIR)/%.o: $(SRCDIR)/%.cpp $(OBJDIR)/.done
 $(OBJDIR)/%.o: $(SRCDIR)/%.c $(OBJDIR)/.done
 	$(CC) -c $(SRC_CFLAGS) $(CFLAGS) -o $@ $<
 
-# TODO: Perhaps detect and use system libdeflate if available.
-maxcso: $(SRC_CXX_OBJ) $(CLI_CXX_OBJ) $(ZOPFLI_C_OBJ) $(SRC_7ZIP) $(SRC_LIBDEFLATE)
-	$(CXX) $(LDFLAGS) -o $@ $(SRC_CXXFLAGS) $(CXXFLAGS) $^ $(LIBS) $(EXTRA_LIBS)
+maxcso: $(OBJS)
+	$(CXX) $(LDFLAGS) -o $@ $(SRC_CXXFLAGS) $(CXXFLAGS) $^ $(LIBS)
 
 $(SRC_7ZIP):
 	$(MAKE) -f $(SRCDIR)/7zip/Makefile 7zip.a