Find Valgrind without pkg-config

[lukaszstolarczuk]

Finding Valgrind package without pkg-config seems problematic, we should deliver FindValgrind.cmake e.g. like this:
https://github.com/elemental/Elemental/blob/master/cmake/modules/FindValgrind.cmake
to enable proper finding using find_package(VALGRIND).

See discussions here: #961 (review)

[lukaszstolarczuk]

We decided we rely on pkg-config. It seems unnecessary to keep also FindValgrind.cmake. In case we have confirmed information this is required (e.g. for some OS) we'll deliver it then.