regex-morph.c: Add alloc_key() under #if HAVE_THREADS_H

[ampli]

See PR #1504. This PR solves the rest of the errors that appear when configuring with `CPPFLAGS="-DHAVE_THREADS_H=0".

[ryandesign]

Thank you, it does compile on macOS with pcre2 with that change, however in make check the multi-dict and multi-thread tests crash with a segmentation fault, which doesn't happen if I disable pcre2 with the --with-regexlib=c configure argument. From the crash log, multi-dict crashed here:

Thread 6 Crashed:
0   libpcre2-8.0.dylib                     0x1029e08a7 match + 49451
1   libpcre2-8.0.dylib                     0x1029d4324 pcre2_match_8 + 4536
2   liblink-grammar.5.dylib                0x102886fbf reg_match + 39 (regex-morph.c:239) [inlined]
3   liblink-grammar.5.dylib                0x102886fbf match_regex + 207 (regex-morph.c:428)
4   liblink-grammar.5.dylib                0x1028b4b06 regex_guess + 12 (tokenize.c:377) [inlined]
5   liblink-grammar.5.dylib                0x1028b4b06 separate_word + 886 (tokenize.c:2681)
6   liblink-grammar.5.dylib                0x1028b4489 separate_sentence + 1257 (tokenize.c:3090)
7   liblink-grammar.5.dylib                0x1028ae13a sentence_split + 74 (sentence.c:93)
8   multi-dict                             0x1026fd5ba parse_one_sent(char const*) + 31 (multi-dict.cc:40) [inlined]
9   multi-dict                             0x1026fd5ba parse_sents(int, int) + 122 (multi-dict.cc:82)
10  multi-dict                             0x1026fd7e0 decltype(static_cast<void (*>(fp)(static_cast<int>(fp0), static_cast<int>(fp0))) std::__1::__invoke<void (*)(int, int), int, int>(void (*&&)(int, int), int&&, int&&) + 4 (type_traits:3918) [inlined]
11  multi-dict                             0x1026fd7e0 void std::__1::__thread_execute<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(int, int), int, int, 2ul, 3ul>(std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(int, int), int, int>&, std::__1::__tuple_indices<2ul, 3ul>) + 4 (thread:287) [inlined]
12  multi-dict                             0x1026fd7e0 void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(int, int), int, int> >(void*) + 48 (thread:298)
13  libsystem_pthread.dylib             0x7ff800d7b4e1 _pthread_start + 125
14  libsystem_pthread.dylib             0x7ff800d76f6b thread_start + 15

while multi-thread crashed here:

Thread 3 Crashed:
0   libpcre2-8.0.dylib                     0x10da218e6 match + 362
1   libpcre2-8.0.dylib                     0x10da21324 pcre2_match_8 + 4536
2   liblink-grammar.5.dylib                0x10d8d3fbf reg_match + 39 (regex-morph.c:239) [inlined]
3   liblink-grammar.5.dylib                0x10d8d3fbf match_regex + 207 (regex-morph.c:428)
4   liblink-grammar.5.dylib                0x10d901b06 regex_guess + 12 (tokenize.c:377) [inlined]
5   liblink-grammar.5.dylib                0x10d901b06 separate_word + 886 (tokenize.c:2681)
6   liblink-grammar.5.dylib                0x10d901489 separate_sentence + 1257 (tokenize.c:3090)
7   liblink-grammar.5.dylib                0x10d8fb13a sentence_split + 74 (sentence.c:93)
8   multi-thread                           0x10d74a233 parse_one_sent(Dictionary_s*, Parse_Options_s*, char const*) + 51 (multi-thread.cc:34)
9   multi-thread                           0x10d74a042 parse_sents(Dictionary_s*, Parse_Options_s*, int, int) + 1378 (multi-thread.cc:125)
10  multi-thread                           0x10d74a445 decltype(static_cast<void (*>(fp)(static_cast<Dictionary_s*>(fp0), static_cast<Parse_Options_s*>(fp0), static_cast<int>(fp0), static_cast<int>(fp0))) std::__1::__invoke<void (*)(Dictionary_s*, Parse_Options_s*, int, int), Dictionary_s*, Parse_Options_s*, int, int>(void (*&&)(Dictionary_s*, Parse_Options_s*, int, int), Dictionary_s*&&, Parse_Options_s*&&, int&&, int&&) + 3 (type_traits:3918) [inlined]
11  multi-thread                           0x10d74a445 void std::__1::__thread_execute<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(Dictionary_s*, Parse_Options_s*, int, int), Dictionary_s*, Parse_Options_s*, int, int, 2ul, 3ul, 4ul, 5ul>(std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(Dictionary_s*, Parse_Options_s*, int, int), Dictionary_s*, Parse_Options_s*, int, int>&, std::__1::__tuple_indices<2ul, 3ul, 4ul, 5ul>) + 17 (thread:287) [inlined]
12  multi-thread                           0x10d74a445 void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(Dictionary_s*, Parse_Options_s*, int, int), Dictionary_s*, Parse_Options_s*, int, int> >(void*) + 53 (thread:298)
13  libsystem_pthread.dylib             0x7ff800d7b4e1 _pthread_start + 125
14  libsystem_pthread.dylib             0x7ff800d76f6b thread_start + 15

1495 should be mentioned in the commit message.

[ampli]

The current regex-morph.c PCRE2 code doesn't support using multi-threading without threads.h.

Possible solutions:

1. Document that, maybe add a warning in configure, and modify the multi-threading tests to print a warning and exit.
2. Modify the regex-morph.c PCRE2 code to use C++ threads, and modify configure.ac and link-grammar/Makefile.am accordingly.
3. Modify the regex-morph.c PCRE2 code to use Pthreads.