style: no longer use backquotes

* README, REFERENCES, TODO, configure.ac, data/README, data/bison.m4,
* data/c++.m4, data/c.m4, data/java.m4, data/lalr1.cc,
* data/lalr1.java, data/yacc.c, doc/local.mk, etc/bench.pl.in,
* src/conflicts.c, src/files.c, src/getargs.c, src/gram.h, src/lalr.c,
* src/location.c, src/location.h, src/muscle-tab.c, src/muscle-tab.h,
* src/output.c, src/parse-gram.c, src/parse-gram.y, src/print-xml.c,
* src/print.c, src/reader.c, src/reduce.c, src/scan-skel.l,
* src/symtab.h, src/system.h, src/tables.c:
Use single quotes, as currently recommended by the GNU Coding Standards.

src/getargs.c

@@ -538,7 +538,7 @@ static struct option const long_options[] =
 };
 
 /* Under DOS, there is no difference on the case.  This can be
-   troublesome when looking for `.tab' etc.  */
+   troublesome when looking for '.tab' etc.  */
 #ifdef MSDOS
 # define AS_FILE_NAME(File) (strlwr (File), (File))
 #else