Oracle has chosen to have the 64 bit version of instantclient install in a different path than the normal version. The difference is the 64 bit has "client64" in the path instead of of "client".
Because of this, the configure script will fail to find the instantclient includes on 64 bit machines:
configure: error: /usr/include/oracle/11.1.0.1/client doesn't exist. Please install the sdk...
2008-07-09 20:14:12 UTC in TOra