package/lcdproc: override libpng-config path

Like for freetype-config, override libpng-config path otherwise
the one from the host system may be used when installed.
Make sure that the one from STAGING_DIR is used.

configure:7480: checking for libpng-config
configure:7498: found /usr/bin/libpng-config
configure:7510: result: /usr/bin/libpng-config
configure:7529: checking whether libpng is present and sane
[...]
nios2-linux-gnu-gcc: ERROR: unsafe header/library path used in cross-compilation: '-I/usr/include/libpng12'

Fixes
http://autobuild.buildroot.net/results/b68/b68f54404a7d412d639ebb6d052412eea6ee3bf1/lcdproc-0.5.7/config.log

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/lcdproc/lcdproc.mk

@@ -22,6 +22,7 @@ LCDPROC_CONF_ENV += ac_cv_path_FT2_CONFIG=$(STAGING_DIR)/usr/bin/freetype-config
 
 ifeq ($(BR2_PACKAGE_LIBPNG),y)
 LCDPROC_DEPENDENCIES += libpng
+LCDPROC_CONF_ENV += ac_cv_path__png_config=$(STAGING_DIR)/usr/bin/libpng-config
 LCDPROC_CONF_OPTS += --enable-libpng
 else
 LCDPROC_CONF_OPTS += --disable-libpng