Improve building the LDAP and OCI8 extensions statically.

Allows the LDAP extension to link with the Oracle Instant Client ZIP
file library LDAP implementation, e.g. using
--with-ldap=/path/to/instantclient.  (Does not support building with
Instant Client RPMS.)

Allows the LDAP and OCI8 extensions to be statically built when not
passing a directory to --with-ldap.  However this uses Oracle ldap.h
headers but doesn't use Oracle's LDAP implementation.  A preferred
user build scenario is to build PHP without OCI8, and then build OCI8
as a shared extension.