After further investigation I found that there is a new TCP_KEEPALIVE constant being declared in IdWinsock2.pas:
{$EXTERNALSYM TCP_KEEPALIVE}
TCP_KEEPALIVE = 3;
This conflicts with the existing tcp_keepalive record, so that has been renamed to _tcp_keepalive. Updating the ka variable declared in TIdStackWindows.SetKeepAliveValues() to this new typename resolves the error.
I then found another compile issue in IdSSLOpenSSL.pas:
[DCC Error] IdSSLOpenSSL.pas(980): E2065 Unsatisfied forward or
external declaration: 'IndySSL_CTX_use_certificate_chain_file'
This error can be resolved by adding a missing implementation of IndySSL_CTX_use_certificate_chain_file() inside the {$ELSE} section of an {$IFDEF STRING_IS_UNICODE} block that starts at line 1155, and have it simply call OpenSSL's standard SSL_CTX_use_certificate_chain_file() function.
UPDATE:
All the necessary fixes (described above) are available in revision 5277 and later.