#89 Fails to build against OpenSSL4

[Finn Gärtner]

During the Ubuntu OpenSSL 4 transition we noticed that isync fails to build against openssl 4:

| gcc -DHAVE_CONFIG_H -I. -I.. -Wdate-time -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -g -O2 -Werror=implicit-function-declaration -ffile-prefix-map=/build/reproducible-path/isync-1.5.1=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -pipe -std=c11 -pedantic -Wall -Wextra -Wshadow -Wcast-qual -Wformat=2 -Wformat-signedness -Wformat-nonliteral -Wstrict-prototypes -Wno-overlength-strings -c -o socket.o socket.c
| socket.c: In function ‘verify_hostname’:
| socket.c:144:71: error: invalid use of incomplete typedef ‘ASN1_IA5STRING’ {aka ‘struct asn1_string_st’}
| 144 | strlen( (const char *)subj_alt_name->d.ia5->data ) == (size_t)subj_alt_name->d.ia5->length &&
| | ^~
| socket.c:144:111: error: invalid use of incomplete typedef ‘ASN1_IA5STRING’ {aka ‘struct asn1_string_st’}
| 144 | strlen( (const char *)subj_alt_name->d.ia5->data ) == (size_t)subj_alt_name->d.ia5->length &&
| | ^~
| socket.c:145:88: error: invalid use of incomplete typedef ‘ASN1_IA5STRING’ {aka ‘struct asn1_string_st’}
| 145 | host_matches( hostname, (const char *)(subj_alt_name->d.ia5->data) ))
| | ^~
| socket.c:157:20: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
| 157 | if (!(subj = X509_get_subject_name( cert ))) {
| | ^
| socket.c:161:9: warning: ‘X509_NAME_get_text_by_NID’ is deprecated: Since OpenSSL 4.0 [-Wdeprecated-declarations]
| 161 | if ((len = X509_NAME_get_text_by_NID( subj, NID_commonName, cname, sizeof(cname) )) < 0) {
| | ^~
| In file included from /usr/include/openssl/ssl.h:34,
| from socket.h:18,
| from socket.c:9:
| /usr/include/openssl/x509.h:1041:27: note: declared here
| 1041 | OSSL_DEPRECATEDIN_4_0 int X509_NAME_get_text_by_NID(const X509_NAME *name,
| | ^~~~~~~~~~~~~~~~~~~~~~~~~
| make[3]: *** [Makefile:735: socket.o] Error 1

Ubuntu bug: https://bugs.launchpad.net/ubuntu/+source/isync/+bug/2154966
Debian bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1138421