[CMake] Include GNUInstallDirs

[jmschonfeld]

The GNUInstallDirs module sets CMAKE_INSTALL_BINDIR which is used on Windows as the destination for installed .dll files. This addresses issues that I saw when building a Windows toolchain where the .dll files were not installed into the toolchain previously, but with this change are correctly installed into the bin directory.

[jmschonfeld]

@swift-ci please test

[itingliu]

For my information, cmake's documentation says this defines MAKE_INSTALL_<dir> where dir can be one of many options, BINDIR being one of those. Does the includer automatically choose BINDIR on Windows or was there another config you had to set to get that to be set?

[jmschonfeld]

In our case, we use it inside of SwiftFoundationSwiftSupport.cmake which has a _swift_foundation_install_target function that installs a swift target (it installs the binary and copies over swift module files). When it installs the target, it installs via

install(TARGETS ${module}
  ARCHIVE DESTINATION lib/${swift}/${swift_os}
  LIBRARY DESTINATION lib/${swift}/${swift_os}
  RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})

and in this case without this include, CMAKE_INSTALL_BINDIR would be empty. On linux this didn't matter since libraries get installed into the library destination path. However on Windows, .dll files get installed into the runtime destination path while .lib files get installed into the library destination path, so with this runtime destination left empty the installed toolchain was missing the .dll files