build: Fix disabling --enable-man if xsltproc is not available
build: Fix disabling --enable-man if xsltproc is not available

If --enable-man=maybe (the default), and xsltproc is not available, the
configuration code would check for it, correctly set enable_man=no, then
incorrectly overwrite that with enable_man=yes, which would result in
later trying to execute $(XSLTPROC) when it’s empty.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #720
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree