Add libpng recipe

recipes/libpng/recipe.sh

+VERSION=1.6.34
+TAR=https://download.sourceforge.net/libpng/libpng-$VERSION.tar.xz
+BUILD_DEPENDS=(zlib)
+
+function recipe_version {
+    echo "$VERSION"
+    skip=1
+}
+
+function recipe_update {
+    echo "skipping update"
+    skip=1
+}
+
+function recipe_build {
+    sysroot="${PWD}/../sysroot"
+    export LDFLAGS="-L$sysroot/lib"
+    export CPPFLAGS="-I$sysroot/include"
+
+    wget -O config.sub http://git.savannah.gnu.org/cgit/config.git/plain/config.sub
+    ./configure --host=${HOST} --prefix='/'
+    make
+    skip=1
+}
+
+function recipe_test {
+    echo "skipping test"
+    skip=1
+}
+
+function recipe_clean {
+    make clean
+    skip=1
+}
+
+function recipe_stage {
+    dest="$(realpath $1)"
+    make DESTDIR="$dest" install
+    skip=1
+}