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/libs/jansson/Makefile

@@ -19,6 +19,8 @@ PKG_LICENSE:=MIT
 PKG_LICENSE_FILES:=LICENSE
 PKG_CPE_ID:=cpe:/a:jansson_project:jansson
 
+PKG_BUILD_FLAGS:=lto
+
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk
 
@@ -41,9 +43,6 @@ define Package/jansson/description
   Jansson is a C library for encoding, decoding and manipulating JSON data
 endef
 
-TARGET_CFLAGS += -flto
-TARGET_LDFLAGS += -flto
-
 define Package/jansson/install
 	$(INSTALL_DIR) $(1)/usr/lib
 	$(CP) $(PKG_INSTALL_DIR)/usr/lib/libjansson*so.* $(1)/usr/lib/

package/libs/libevent2/Makefile

@@ -26,7 +26,7 @@ PKG_CONFIG_DEPENDS:= \
 	CONFIG_PACKAGE_libevent2-pthreads \
 	CONFIG_PACKAGE_libevent2-mbedtls
 
-PKG_BUILD_FLAGS:=gc-sections
+PKG_BUILD_FLAGS:=gc-sections lto
 
 include $(INCLUDE_DIR)/package.mk
 include $(INCLUDE_DIR)/cmake.mk
@@ -114,8 +114,8 @@ define Package/libevent2-pthreads/description
 	threading & locking.
 endef
 
-TARGET_CFLAGS += $(FPIC) -flto
-TARGET_LDFLAGS += -Wl,--as-needed -flto
+TARGET_CFLAGS += $(FPIC)
+TARGET_LDFLAGS += -Wl,--as-needed
 
 CMAKE_OPTIONS += \
 	-DEVENT__DISABLE_BENCHMARK:BOOL=ON \

package/libs/libnftnl/Makefile

@@ -22,6 +22,7 @@ PKG_LICENSE_FILES:=COPYING
 
 PKG_INSTALL:=1
 PKG_BUILD_PARALLEL:=1
+PKG_BUILD_FLAGS:=lto
 
 include $(INCLUDE_DIR)/package.mk
 
@@ -41,8 +42,7 @@ define Package/libnftnl/description
  programming interface (API) to the in-kernel nf_tables subsystem.
 endef
 
-TARGET_CFLAGS += $(FPIC) -flto
-TARGET_LDFLAGS += -flto
+TARGET_CFLAGS += $(FPIC)
 
 CONFIGURE_ARGS += \
 	--enable-static \

package/libs/wolfssl/Makefile

@@ -17,7 +17,7 @@ PKG_HASH:=b7ee150e49def77c765bc02aac92ddeb0bebefd4cb12aa263d8f95e405221fb8
 
 PKG_FIXUP:=libtool libtool-abiver
 PKG_INSTALL:=1
-PKG_BUILD_FLAGS:=no-mips16
+PKG_BUILD_FLAGS:=no-mips16 lto
 PKG_BUILD_PARALLEL:=1
 PKG_LICENSE:=GPL-2.0-or-later
 PKG_LICENSE_FILES:=LICENSING COPYING
@@ -118,12 +118,9 @@ endef
 TARGET_CFLAGS += \
 	$(FPIC) \
 	-fomit-frame-pointer \
-	-flto \
 	-DFP_MAX_BITS=8192 \
 	$(if $(CONFIG_WOLFSSL_ALT_NAMES),-DWOLFSSL_ALT_NAMES)
 
-TARGET_LDFLAGS += -flto
-
 # --enable-stunnel needed for OpenSSL API compatibility bits
 CONFIGURE_ARGS += \
 	--enable-reproducible-build \