pm: do not quote `like this', as per GCS recommendation

pm: do not quote `like this', as per GCS recommendation

This patch converts the internal perl modules used by automake and
aclocal to the use of new quoting format 'like this' or "like this"
rather than `like this'.

* lib/Automake/ChannelDefs.pm, lib/Automake/Channels.pm,
lib/Automake/Condition.pm, lib/Automake/Configure_ac.pm,
lib/Automake/DisjConditions.pm, lib/Automake/FileUtils.pm,
lib/Automake/General.pm, lib/Automake/Item.pm,
lib/Automake/Rule.pm, lib/Automake/Getopt.pm,
lib/Automake/Options.pm, lib/Automake/Struct.pm,
lib/Automake/VarDef.pm, lib/Automake/Variable.pm,
lib/Automake/Version.pm, lib/Automake/XFile.pm: Update and
adjust quoting format throughout, in comments and diagnostic.
Some minor related rewordings and reformatting since we are
at it.