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/location.c

@@ -219,7 +219,7 @@ void
 boundary_set_from_string (boundary *bound, char *loc_str)
 {
   /* Must search in reverse since the file name field may
-   * contain `.' or `:'.  */
+   * contain '.' or ':'.  */
   char *delim = strrchr (loc_str, '.');
   aver (delim);
   *delim = '\0';