Allow building gpgkeys_ldap with the 32 bit mingw-w64 toolchain.
* keyserver/gpgkeys_ldap.c (my_ldap_start_tls_s): Define macro depending on compiler version. (main): Use new macro.
It seems that the LDAP keyserver helper if build with the old mingw32
toolchain never worked correctly for LDAPS. The prototype there for
ldap_start_tls_s is plainly wrong for Windows. However, the fix here
has special support so not to break building with the old compiler.