BUILD: add USE_WURFL to the list of known build options

Since the removal of WURFL we've improved the build system to report
known build options, let's reference this option there as well.
diff --git a/Makefile b/Makefile
index e7a9db0..dd5274f 100644
--- a/Makefile
+++ b/Makefile
@@ -282,7 +282,7 @@
            USE_LINUX_SPLICE USE_LIBCRYPT USE_CRYPT_H USE_VSYSCALL             \
            USE_GETADDRINFO USE_OPENSSL USE_LUA USE_FUTEX USE_ACCEPT4          \
            USE_MY_ACCEPT4 USE_ZLIB USE_SLZ USE_CPU_AFFINITY USE_TFO USE_NS    \
-           USE_DL USE_RT USE_DEVICEATLAS USE_51DEGREES USE_SYSTEMD            \
+           USE_DL USE_RT USE_DEVICEATLAS USE_51DEGREES USE_WURFL USE_SYSTEMD  \
            USE_OBSOLETE_LINKER USE_PRCTL
 
 #### Target system options
@@ -632,14 +632,13 @@
 WURFL_INC = $(WURFL_SRC)
 WURFL_LIB = $(WURFL_SRC)
 OPTIONS_OBJS    += src/wurfl.o
-OPTIONS_CFLAGS  += -DUSE_WURFL $(if $(WURFL_INC),-I$(WURFL_INC))
+OPTIONS_CFLAGS  += $(if $(WURFL_INC),-I$(WURFL_INC))
 ifneq ($(WURFL_DEBUG),)
 OPTIONS_CFLAGS  += -DWURFL_DEBUG
 endif
 ifneq ($(WURFL_HEADER_WITH_DETAILS),)
 OPTIONS_CFLAGS  += -DWURFL_HEADER_WITH_DETAILS
 endif
-BUILD_OPTIONS   += $(call ignore_implicit,USE_WURFL)
 OPTIONS_LDFLAGS += $(if $(WURFL_LIB),-L$(WURFL_LIB)) -lwurfl
 endif