Only test mkl library with --with-libmkl

If we do not make the test conditionally and use --without-libmkl during configure,
we end up with -Lno in LDFLAGS.

Signed-off-by: Justin Lecher <jlec@gentoo.org>
Update logic for mkl location testing

Signed-off-by: Justin Lecher <jlec@gentoo.org>
Merge pull request #18 from jlec/master

Only test mkl library with --with-libmkl
Merge branch 'master' into hotfix/boost1.49-boost_system