package/netsurf: use HOST_CFLAGS/HOST_LDFLAGS

package/netsurf: use HOST_CFLAGS/HOST_LDFLAGS

In commit 9b0ac875385e3bcb052fc2f376f45fa61e102673 ("package/netsurf:
make sure host libpng can be found"), the netsurf package was modified
to pass -L$(HOST_DIR)/lib so that libpng is found at link time.

However, this is not sufficient: for the host tool to work at runtime,
we need to have the proper RPATH encoded in the host
binary. Otherwise, building netsurf fails with:

build/Linux-framebuffer/tools/convert_image: error while loading shared libraries: libpng16.so.16: cannot open shared object file: No such file or directory
make[3]: *** [build/Linux-framebuffer/image-caret_image.c] Error 127

So basically, we need to build not only with -L$(HOST_DIR)/lib, but
with the complete HOST_LDFLAGS provided by Buildroot.

For consistency, we also use HOST_CFLAGS instead of hardcoding
-I$(HOST_DIR)/include.

It is worth mentioning that we must use single quotes here, because
the NETSURF_CONFIG variable value then gets put within double quotes
to be passed to the netsurf build system.

Fixes:

  http://autobuild.buildroot.net/results/4ff6a8c4017d006a4b6b9ca369a569fa72862900/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>