Overrides the linkers used for kvazaar, libkvazaar.la and kvazaar_tests.
When crypto++ is enabled, the C++ linker is used and when it is
disabled, the C linker is used.
This removes the need to explicitly specify -lstdc++ in configure when
crypto++ is used and fixes the build with crypto++ when libstd++ is not
installed.
Moves tests from .travis.yml to bash scripts in tests directory. Adds
the test scripts to tests/Makefile.am so that they are included when
running make check.