cleanup CMAKE lib vars

[bratpiorka]

Today we define set of lib-related CMAKE vars like:

-- Will fetch jemalloc from https://github.com/jemalloc/jemalloc.git (tag: 5.3.0)
--     JEMALLOC_LIBRARIES = /home/rrudnick/unified-memory-framework/build/_deps/jemalloc_targ-build/lib/libjemalloc.a
--     JEMALLOC_INCLUDE_DIRS = /home/rrudnick/unified-memory-framework/build/_deps/jemalloc_targ-build/include
--     JEMALLOC_LIBRARY_DIRS = /home/rrudnick/unified-memory-framework/build/_deps/jemalloc_targ-build/lib

the problem is that we mix *_LIBRARIES with *_LIBRARY_DIRS - we should have full path defined only in *_LIBRARY_DIRS.
Additionally, all vars defined in UMF should have an "UMF" prefix.

[PatKamin]

I think we should have all vars defined like UMF_*_LIBRARY, ie. in a singular form which is a common standard. UMF prefix should be enough to avoid some possible(?) issues in case UMF is built as a subproject.

[PatKamin]

We should rather stick with *_LIBRARY rather than *_LIBRARY_DIR, as this is the common practice for defining the path to a library in CMake. *_LIBRARY_DIR could be retrieved from the full path in-place, as needed.

[bratpiorka]

I would prefer having args that could be directly mapped to target_link_directories() and target_include_directories()