flex, libpng: Use $(SOURCEFORGE_MIRROR)

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

src/flex.mk

@@ -7,7 +7,7 @@ $(PKG)_VERSION  := 2.5.37
 $(PKG)_CHECKSUM := db4b140f2aff34c6197cab919828cc4146aae218
 $(PKG)_SUBDIR   := flex-$($(PKG)_VERSION)
 $(PKG)_FILE     := flex-$($(PKG)_VERSION).tar.bz2
-$(PKG)_URL      := http://prdownloads.sourceforge.net/flex/$($(PKG)_FILE)
+$(PKG)_URL      := http://$(SOURCEFORGE_MIRROR)/project/flex/$($(PKG)_FILE)
 $(PKG)_DEPS     :=
 
 define $(PKG)_UPDATE

src/libpng.mk

@@ -7,7 +7,7 @@ $(PKG)_VERSION  := 1.6.13
 $(PKG)_CHECKSUM := 5ae32b6b99cef6c5c85feab8edf9d619e1773b15
 $(PKG)_SUBDIR   := libpng-$($(PKG)_VERSION)
 $(PKG)_FILE     := libpng-$($(PKG)_VERSION).tar.xz
-$(PKG)_URL      := http://downloads.sourceforge.net/project/libpng/libpng16/$($(PKG)_VERSION)/$($(PKG)_FILE)
+$(PKG)_URL      := http://$(SOURCEFORGE_MIRROR)/project/libpng/libpng16/$($(PKG)_VERSION)/$($(PKG)_FILE)
 $(PKG)_URL_2    := ftp://ftp.simplesystems.org/pub/$(PKG)/png/src/libpng16/$($(PKG)_FILE)
 $(PKG)_DEPS     := gcc zlib