Problem emerging firefox-10.0.11 with libpng

Problem :

I had the following problem when I builded firefox 10.0.11 :

[i]checking for png_get_acTL in -lpng... yes
configure: error: --with-system-png requested but no working libpng found
*** Fix above errors and then restart with               "make -f client.mk build"
make[2]: *** [configure] Error 1
make[2]: Leaving directory `/var/tmp/portage/www-client/firefox-10.0.11/work/mozilla-esr10'
make[1]: *** [obj-i686-pc-linux-gnu/Makefile] Error 2
make[1]: Leaving directory `/var/tmp/portage/www-client/firefox-10.0.11/work/mozilla-esr10'
make: *** [build] Error 2
emake failed
 * ERROR: www-client/firefox-10.0.11 failed (compile phase):
 *   emake failed
 *
 * Call stack:
 *     ebuild.sh, line  93:  Called src_compile
 *   environment, line 7029:  Called die
 * The specific snippet of code:
 *           CC="$(tc-getCC)" CXX="$(tc-getCXX)" LD="$(tc-getLD)" MOZ_MAKE_FLAGS="${MAKEOPTS}" emake -f client.mk || die "emake failed";
 *
 * If you need support, post the output of `emerge --info '=www-client/firefox-10.0.11'`,
 * the complete build

Solution :

There were a conflict with an old version of libpng.To solve the problem, I had to delete libpng files in  /usr/local/include and /usr/local/lib. Thanks to ssuominen for the solution :  http://forums.gentoo.org/viewtopic-p-7215412.html#7215412