This reverts commit b63029c8

This might be a bug in gb_Helper_optional or some such, but I get:

make[1]: *** No rule to make target `/home/master/src/libreoffice/workdirs/master/src/9e5d864bce8f06751bbd99962ecf4aad-libpng-1.5.10.tar.gz', needed by `/home/master/src/libreoffice/workdirs/master/workdir/unxlngx6/UnpackedTarget/9e5d864bce8f06751bbd99962ecf4aad-libpng-1.5.10.tar.gz'.  Stop.
make[1]: *** Waiting for unfinished jobs....
make: *** [build] Error 2

Change-Id: Ib9cd0cbe800f158bf50aaeb95f848cc2fd805de3
Reviewed-on: https://gerrit.libreoffice.org/6287Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Lionel Elie Mamane <lionel@mamane.lu>