use GNUInstallDirs for install paths

[elelayan]

to be able to install libraptor2.a and raptor2.pc to a specific directory when cross-compiling

[dajobe]

cmake is primarily for building on windows, so does this change using cmake on windows? I don't use it so can't test that.

[elelayan]

As far as I can tell it doesn't change anything on windows: GNUInstallDirs defaults

• CMAKE_INSTALL_INCLUDEDIR to include,
• CMAKE_INSTALL_BINDIR to bin,
• CMAKE_INSTALL_LIBDIR to lib,
• CMAKE_INSTALL_MANDIR to share/man.

The only change: CMAKE_INSTALL_LIBDIR is lib64 when on Linux (not Debian) 64bits when not cross-compiling.

The point is to be able to override these variables on the command-line (e.g. pass -DCMAKE_INSTALL_LIBDIR=libarm to cmake).