Makefile: remove .SILENT, make the targets loud

[Flowdalic]

The Makefile was set globally to silent, which caused the failing commands to be not displayed. For example

flo@neo-pc xeps-xsf $ make html
make: *** [Makefile:112: build/xep-0001.html] Error 1

Removing this provides usefull information about which command failed:

flo@neo-pc xeps-xsf $ make html
xmllint --nonet --noout --noent --loaddtd --valid "xep-0001.xml" ! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" xep-0001.xml 2>/dev/null && true make: *** [Makefile:110: build/xep-0001.html] Error 1

[deuill]

Seems that the underlying issue here is the same as encountered in #1316, were you able to solve these somehow @Flowdalic?