backend/Makefile: Fix installation of backend libraries

When using Automake variables like sanelib* to install files in a custom directory, the files are assumed to be platform-independent, unless these variables contain exec in the name. This affects whether the files are installed during make install-data or make install-exec. It does not matter whether a suffix like _DATA or _LTLIBRARIES is added to this name.

The packaging scripts for Debian call those Makefile targets separately and are affected by this behavior. Since the backend libraries themselves are platform-dependent files, rename these variables to execsanelib*.