This is error:

  CC       libcurl_la-openldap.lo
openldap.c:83:17: error: conflicting types for 'ldap_connect'
   83 | static CURLcode ldap_connect(struct Curl_easy *data, bool *done);
      |                 ^~~~~~~~~~~~
In file included from openldap.c:39:
/pz/SFW/include/ldap.h:1554:1: note: previous declaration of 'ldap_connect' was here
 1554 | ldap_connect( LDAP *ld );
      | ^~~~~~~~~~~~
gmake[2]: *** [Makefile:2515: libcurl_la-openldap.lo] Error 1

PREFIX is where new OpenLDAP is installed (as well as OpenSSL)
PKG_CONFIG_PATH=${PREFIX}/lib/pkgconfig CPPFLAGS="-I${PREFIX}/include -D_POSIX_PTHREAD_SEMANTICS -D_XOPEN_SOURCE=600 -D_FILE_OFFSET_BITS=64 -D__EXTENSIONS__" CFLAGS="-m64 -march=native -std=c89 -std=c99" LDFLAGS="-L${PREFIX}/lib -R${PREFIX}/lib" LIBS="-lstdc++" ./configure --prefix=${PREFIX} --enable-optimize --enable-http --enable-ftp --enable-file --enable-proxy --enable-telnet --enable-tftp --enable-manual --enable-libcurl-option --enable-ipv6 --enable-threaded-resolver --enable-crypto-auth --enable-cookies --with-libssh2 --with-gssapi=${PREFIX} --with-ssl=${PREFIX} --with-ca-fallback --enable-ldap --enable-ldaps --with-ca-bundle=/etc/openssl/ca-bundle.crt --with-ca-path=/etc/openssl/certs
OS is OpenIndiana (Illumos based), GCC 9 suite used. Same configure works and curl compiles fine with OpenLDAP 2.4.xx.

Regards.

Yeah, turns out we use ldap_ prefixes on private functions in that source file and OpenLDAP has introduced a function that uses the exact same name as we use for a private function. The fix is to avoid that prefix for private functions.

FYI:
it compiles now (tested against git clone):

:; curl --version
curl 7.77.0-DEV (x86_64-pc-solaris2.11) libcurl/7.77.0-DEV OpenSSL/1.1.1k zlib/1.2.11 brotli/1.0.9 zstd/1.4.10 libidn2/2.3.0 libssh2/1.9.0 nghttp2/1.37.0 librtmp/2.3 libgsasl/1.8.1
Release-Date: [unreleased]
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli gsasl GSS-API HSTS HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB SPNEGO SSL TLS-SRP UnixSockets zstd

Many thanks