package/pngquant: fix target build

package/pngquant: fix target build

Pngquant 2.10 introduced a bundled library libimagequant, Makefile
passes $LIQCONFIGUREFLAGS to the configure script of libimagequant
before building this library:
https://github.com/pornel/pngquant/blob/master/Makefile#L27

This variable is filled by configure with the content of $LIQCONFIGURE
https://github.com/pornel/pngquant/blob/master/configure#L553

We need to pass CC/CFLAGS/LDFLAGS as parameter to configure to fill
$LIQCONFIGURE with correct values in order to use the target toolchain
for building libimagequant:
https://github.com/pornel/pngquant/blob/master/configure#L56

Fixes
http://autobuild.buildroot.net/results/ab1/ab1193db9d76adc02be44d6f273e0986952df3e2/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>