am: do not quote `like this', as per GCS recommendation

am: do not quote `like this', as per GCS recommendation

This patch converts the automake-provided '*.am' fragments, and
related files, to the use of new quoting format 'like this' or
"like this" rather than `like this'.

* lib/am/check.am: Update quoting format throughout, in comments
and diagnostic.  Some related rewordings, reformatting, and removal
of redundant commands since we are at it.
* lib/am/configure.am: Likewise.
* lib/am/dejagnu.am: Likewise.
* lib/am/depend2.am: Likewise.
* lib/am/distdir.am: Likewise.
* lib/am/inst-vars.am: Likewise.
* lib/am/install.am: Likewise.
* lib/am/lang-compile.am: Likewise.
* lib/am/lisp.am: Likewise.
* lib/am/ltlib.am: Likewise.
* lib/am/mans.am: Likewise.
* lib/am/progs.am: Likewise.
* lib/am/remake-hdr.am: Likewise.
* lib/am/subdirs.am: Likewise.
* lib/am/tags.am: Likewise.
* lib/am/texi-vers.am: Likewise.
* lib/am/texibuild.am: Likewise.
* lib/am/texinfos.am: Likewise.
* lib/am/yacc.am: Likewise.