--without-system-libpng for "universal" Linux builds

...so that e.g. Fedora 17 does not require installation of libpng-compat package
for old libpng12.so.

Change-Id: I45cf3928b30dbbb815f70cde1972dc4972007c05

distro-configs/LibreOfficeLinux.conf

@@ -7,6 +7,7 @@
 --without-system-openssl
 --without-system-mozilla
 --without-system-mesa-headers
+--without-system-libpng
 --without-system-libxml
 --without-system-jpeg
 --without-system-jars