See http://bugs.php.net/bug.php?id=39312.

--- a/ext/pdo_oci/config.m4.orig	2009-01-31 18:26:43.000000000 +0100
+++ b/ext/pdo_oci/config.m4	2009-01-31 18:28:46.000000000 +0100
@@ -97,11 +97,11 @@
     else
       AC_MSG_ERROR([I'm too dumb to figure out where the include dir is in your Instant Client install])
     fi
-    if test -f "$PDO_OCI_IC_PREFIX/lib/oracle/$PDO_OCI_IC_VERS/client/lib/libclntsh.so" ; then
+    if test -f "$PDO_OCI_IC_PREFIX/lib/oracle/$PDO_OCI_IC_VERS/client/lib/libclntsh.$SHLIB_SUFFIX_NAME" ; then
       PDO_OCI_LIB_DIR="$PDO_OCI_IC_PREFIX/lib/oracle/$PDO_OCI_IC_VERS/client/lib"
-    elif test -f "$PDO_OCI_IC_PREFIX/client/lib/libclntsh.so" ; then
+    elif test -f "$PDO_OCI_IC_PREFIX/client/lib/libclntsh.$SHLIB_SUFFIX_NAME" ; then
       PDO_OCI_LIB_DIR="$PDO_OCI_IC_PREFIX/client/lib"
-    elif test -f "$PDO_OCI_IC_PREFIX/libclntsh.so" ; then
+    elif test -f "$PDO_OCI_IC_PREFIX/libclntsh.$SHLIB_SUFFIX_NAME" ; then
       PDO_OCI_LIB_DIR="$PDO_OCI_IC_PREFIX"
     else
       AC_MSG_ERROR([I'm too dumb to figure out where the libraries are in your Instant Client install])
