The libclntsh.a is an "archived" file, which cannot be opened by our driver. The way around this is to "unarchive" this file and extract the shared object. This can be done by changing directory to $ORACLE_HOME/lib ($ORACLE_HOME/lib32 if you are not using the 64-bit library) and typing ar -xv libclntsh.a
.
This will then generate a file called shr.o which will then need to be renamed to libclntsh.so. The driver will then be able to load the required shared object.
Please then make sure that $ORACLE_HOME/lib (/lib32) is added to your LIBPATH.