Configuring C++ library to use MKL

I'm trying to configure Opt++ to use MKL on Debian 8. Their document (https://software.sandia.gov/opt++/opt++2.4_doc/html/InstallDoc.html#blas) indicates to use the syntax

./configure --with-blas=/path/to/blas/blaslibraryname.a

however, when I checked the following two paths, there are several .a files

/opt/intel/mkl/lib/intel64/

/opt/intel/compilers_and_libraries/linux/mkl/lib/intel64

Which library do I use? I tried a couple, but it doesn't appear to work according to the configure.log, which points out undefined reference to sgemm_.

Thanks.