m4: do not quote `like this', as per GCS recommendation

m4: do not quote `like this', as per GCS recommendation

This patch converts the files comprising the automake's own build
system to the use of new quoting format 'like this' or "like this"
rather than `like this'.

* m4/auxdir.m4, m4/ccstdc.m4, m4/depend.m4, m4/depout.m4, m4/tar.m4,
m4/init.m4, m4/maintainer.m4, m4/make.m4, m4/missing.m4, m4/vala.m4,
m4/mkdirp.m4, m4/sanity.m4, m4/silent.m4, m4/strip.m4: Update and
adjust quoting format throughout, in comments and diagnostic.  Some
minor related rewordings and reformatting since we are at it.  Bump
serial numbers.