Use GNUInstallDirs variables for CMake install destinations

[hesiod]

This PR replaces bare relative install destinations like bin or lib with their corresponding variables from GNUInstallDirs (see module documentation). The variables should default to the same values as previously used (other than CMAKE_INSTALL_DOCDIR, for which we override the default). For example, CMAKE_INSTALL_BINDIR defaults to bin.

Using variables from GNUInstallDirs can simplify packaging Geogram in some cases as it allows overriding install destinations for certain file types. Using GNUInstallDirs variables is commonly considered a good practice (at least to my knowledge).

There are no standard variables for CMake and pkg-config files. Therefore this PR adds GEOGRAM_INSTALL_CMAKE_DIR and GEOGRAM_INSTALL_PKGCONFIG_DIR.