* Reproducible builds! No more absolute paths in source files, so the
  generated binary is also invariant of the build directory.
* Output modules.h in snowball/libstemmer/ instead of snowball/. This
  allows removal of the extra include directory and -f option.
* This partially reverts some changes in
  d178f201fd
  and matches the commands in GNUmakefile.

../contrib/snowball/compiler/analyser.c: In function ‘check_name_type’:
../contrib/snowball/compiler/analyser.c:210:19: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
  210 |         case 'r': if (p->type == t_routine ||
      |                   ^~
../contrib/snowball/compiler/analyser.c:211:54: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
  211 |                       p->type == t_external) return; break;
      |                                                      ^~~~~
../contrib/snowball/compiler/analyser.c: In function ‘read_program’:
../contrib/snowball/compiler/analyser.c:859:21: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
  859 |                     if (q->used && q->definition == 0) error4(a, q); break;
      |                     ^~
../contrib/snowball/compiler/analyser.c:859:70: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
  859 |                     if (q->used && q->definition == 0) error4(a, q); break;
      |                                                                      ^~~~~
../contrib/snowball/compiler/analyser.c:861:21: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
  861 |                     if (q->used && q->grouping == 0) error4(a, q); break;
      |                     ^~
../contrib/snowball/compiler/analyser.c:861:68: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
  861 |                     if (q->used && q->grouping == 0) error4(a, q); break;
      |                                                                    ^~~~~