Make dependencies' *_OBJS variables overridable via environment variables.

Makefile

@@ -1,9 +1,9 @@
 OUT=webdis
-HIREDIS_OBJ=hiredis/hiredis.o hiredis/sds.o hiredis/net.o hiredis/async.o
-JANSSON_OBJ=jansson/src/dump.o jansson/src/error.o jansson/src/hashtable.o jansson/src/load.o jansson/src/strbuffer.o jansson/src/utf.o jansson/src/value.o jansson/src/variadic.o
-B64_OBJS=b64/cencode.o
-FORMAT_OBJS=formats/json.o formats/raw.o formats/common.o formats/custom-type.o
-HTTP_PARSER_OBJS=http-parser/http_parser.o
+HIREDIS_OBJ?=hiredis/hiredis.o hiredis/sds.o hiredis/net.o hiredis/async.o
+JANSSON_OBJ?=jansson/src/dump.o jansson/src/error.o jansson/src/hashtable.o jansson/src/load.o jansson/src/strbuffer.o jansson/src/utf.o jansson/src/value.o jansson/src/variadic.o
+B64_OBJS?=b64/cencode.o
+FORMAT_OBJS?=formats/json.o formats/raw.o formats/common.o formats/custom-type.o
+HTTP_PARSER_OBJS?=http-parser/http_parser.o
 
 CFLAGS ?= -O0 -ggdb -Wall -Wextra -I. -Ijansson/src -Ihttp-parser
 LDFLAGS ?= -levent -pthread