Use GNUInstallDirs; Fix install location of CMake config files; include CMakeFindDependencyMacro module

[topazus]

@yixuan The PR does the following things:

• Use GNUInstallDirs. There are some reasonable variables that can be used to faciliate the installation in CMake, instead of the hard-coded path.
• Fix install location of CMake config files. It should be ${CMAKE_INSTALL_DATADIR}/cmake/Spectra not share/spectra/cmake, in Linux as I know.
• Add include(CMakeFindDependencyMacro) before using find_dependency. https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html