This doesn't apply to every system. For those systems on which it
does apply, their tooling should be setting CMAKE_INSTALL_RPATH to
their system-global locations.
* CMakeLists.txt (CMAKE_INSTALL_RPATH): Remove /usr/${SD_LIBDIR}/lib.
The <SOLID/solid.h> header would not be made available when using a
pkg-config discovered FreeSOLID library.
* cmake/FindSOLID.cmake: Add IMPORTED_TARGET argument to
'pkg_check_modules' calls.
[PKGCONFIG_FOUND && SOLID_FOUND]: Alias 'solid' to PkgConfig::SOLID.
Fixes: #45
