treewide: add support for "lto" in PKG_BUILD_FLAGS

This reduces open coding and allows to easily add a knob to enable
it treewide, where chosen packages can still opt-out via "no-lto".

Some packages used LTO, but not the linker plugin. This unifies 'em
all to attempt to produce better code.
Quoting man gcc(1):
"This improves the quality of optimization by exposing more code to the
link-time optimizer."

Also use -flto=auto instead of -flto=jobserver, as it's not guaranteed
that every buildsystem uses +$(MAKE) correctly.

Signed-off-by: Andre Heider <a.heider@gmail.com>

package/network/services/dnsmasq/Makefile

@@ -24,6 +24,7 @@ PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_UPSTR
 
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=lto
 PKG_ASLR_PIE_REGULAR:=1
 PKG_CONFIG_DEPENDS:= CONFIG_PACKAGE_dnsmasq_$(BUILD_VARIANT)_dhcp \
 	CONFIG_PACKAGE_dnsmasq_$(BUILD_VARIANT)_dhcpv6 \
@@ -133,9 +134,6 @@ endef
 Package/dnsmasq-dhcpv6/conffiles = $(Package/dnsmasq/conffiles)
 Package/dnsmasq-full/conffiles = $(Package/dnsmasq/conffiles)
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver
-
 COPTS = -DHAVE_UBUS -DHAVE_POLL_H \
 	$(if $(CONFIG_IPV6),,-DNO_IPV6)
 

package/network/services/dropbear/Makefile

@@ -23,7 +23,7 @@ PKG_CPE_ID:=cpe:/a:matt_johnston:dropbear_ssh_server
 
 PKG_BUILD_PARALLEL:=1
 PKG_ASLR_PIE_REGULAR:=1
-PKG_BUILD_FLAGS:=no-mips16 gc-sections
+PKG_BUILD_FLAGS:=no-mips16 gc-sections lto
 PKG_FIXUP:=autoreconf
 PKG_FLAGS:=nonshared
 
@@ -142,8 +142,7 @@ DB_OPT_CONFIG = \
 	DROPBEAR_SVR_AGENTFWD|CONFIG_DROPBEAR_AGENTFORWARD|1|0 \
 
 
-TARGET_CFLAGS += -DARGTYPE=3 -flto
-TARGET_LDFLAGS += -flto=jobserver
+TARGET_CFLAGS += -DARGTYPE=3
 
 db_opt_add     =echo '\#define $(1) $(2)' >> $(PKG_BUILD_DIR)/localoptions.h
 db_opt_replace =$(ESED) 's,^(\#define $(1)) .*$$$$,\1 $(2),g' $(PKG_BUILD_DIR)/sysoptions.h

package/network/services/hostapd/Makefile

@@ -32,7 +32,7 @@ PKG_CONFIG_DEPENDS:= \
 	CONFIG_DRIVER_11AX_SUPPORT \
 	CONFIG_WPA_ENABLE_WEP
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 EAPOL_TEST_PROVIDERS:=eapol-test eapol-test-openssl eapol-test-wolfssl
 
@@ -587,8 +587,7 @@ TARGET_CPPFLAGS := \
 	-D_GNU_SOURCE \
 	$(if $(CONFIG_WPA_MSG_MIN_PRIORITY),-DCONFIG_MSG_MIN_PRIORITY=$(CONFIG_WPA_MSG_MIN_PRIORITY))
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto=jobserver -fuse-linker-plugin -lubox -lubus
+TARGET_LDFLAGS += -lubox -lubus
 
 ifdef CONFIG_PACKAGE_kmod-cfg80211
   TARGET_LDFLAGS += -lm -lnl-tiny

package/network/services/lldpd/Makefile

@@ -21,6 +21,7 @@ PKG_CPE_ID:=cpe:/a:lldpd_project:lldpd
 
 PKG_FIXUP:=autoreconf
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=lto
 PKG_INSTALL:=1
 
 include $(INCLUDE_DIR)/package.mk
@@ -111,7 +112,6 @@ CONFIGURE_ARGS += \
 	$(if $(CONFIG_LLDPD_WITH_SNMP),--with-snmp,) \
 	$(if $(CONFIG_USE_GLIBC),,--without-libbsd)
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto -Wl,--gc-sections,--as-needed
+TARGET_LDFLAGS += -Wl,--gc-sections,--as-needed
 
 $(eval $(call BuildPackage,lldpd))

package/network/services/ppp/Makefile

@@ -26,7 +26,7 @@ PKG_VERSION:=$(PKG_RELEASE_VERSION).git-$(PKG_SOURCE_DATE)
 PKG_BUILD_DEPENDS:=libpcap
 
 PKG_ASLR_PIE_REGULAR:=1
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 PKG_BUILD_PARALLEL:=1
 PKG_INSTALL:=1
 
@@ -193,9 +193,6 @@ $(call Build/Configure/Default,, \
 		$(PKG_BUILD_DIR)/pppd/plugins/pppoatm/linux/
 endef
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto -fuse-linker-plugin
-
 MAKE_FLAGS += COPTS="$(TARGET_CFLAGS)" \
 		PRECOMPILED_FILTER=1 \
 		STAGING_DIR="$(STAGING_DIR)"

package/network/services/ustp/Makefile

@@ -19,6 +19,8 @@ PKG_MIRROR_HASH:=0e96edc983cf437b95874e5715d743f30bb826d8757dc3771ff872ab9cf18f3
 PKG_MAINTAINER:=Felix Fietkau <nbd@nbd.name>
 PKG_LICENSE:=GPL-2.0
 
+PKG_BUILD_FLAGS:=lto
+
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk
 
@@ -29,8 +31,7 @@ define Package/ustp
   DEPENDS:=+libubox +libubus
 endef
 
-TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include -flto
-TARGET_LDFLAGS += -flto -fuse-linker-plugin
+TARGET_CFLAGS += -I$(STAGING_DIR)/usr/include
 
 define Package/ustp/install
 	$(INSTALL_DIR) $(1)/sbin $(1)/etc/init.d