diagnostics: suggest fixes for undeclared symbols

From

    input.y:1.17-19: warning: symbol baz is used, but is not defined as a token and has no rules [-Wother]
         1 | %printer {} foo baz
           |                 ^~~

to

    input.y:1.17-19: warning: symbol 'baz' is used, but is not defined as a token and has no rules; did you mean 'bar'? [-Wother]
        1 | %printer {} foo baz
          |                 ^~~
          |                 bar

* bootstrap.conf: We need fstrcmp.
* src/symtab.c (symbol_from_uniqstr_fuzzy): New.
(complain_symbol_undeclared): Use it.
* tests/diagnostics.at (Suggestions): New.
* data/bison-default.css (insertion): Rename as...
(fixit-insert): this, as this is what GCC uses.

lib/.gitignore

@@ -53,6 +53,7 @@
 /config.h
 /config.in.h
 /configmake.h
+/diffseq.h
 /dirname-lgpl.c
 /dirname.c
 /dirname.h
@@ -91,6 +92,8 @@
 /fseterr.c
 /fseterr.h
 /fstat.c
+/fstrcmp.c
+/fstrcmp.h
 /fsync.c
 /getdtablesize.c
 /gethrxtime.c
@@ -324,6 +327,8 @@
 /windows-recmutex.h
 /windows-rwlock.c
 /windows-rwlock.h
+/windows-tls.c
+/windows-tls.h
 /xalloc-die.c
 /xalloc-oversized.h
 /xalloc.h

lib/glthread/.gitignore

@@ -1,3 +1,5 @@
 /lock.c
 /lock.h
 /threadlib.c
+/tls.c
+/tls.h