Use GNUInstallDirs for install destinations

This has the advantage that the user may override e.g. the include
location, and the correct libdir (lib, lib64, lib/something) is
automatically determined.

Closes #1367.

CMakeLists.txt

@@ -14,7 +14,6 @@ set(GLFW_VERSION_PATCH "0")
 set(GLFW_VERSION_EXTRA "")
 set(GLFW_VERSION "${GLFW_VERSION_MAJOR}.${GLFW_VERSION_MINOR}")
 set(GLFW_VERSION_FULL "${GLFW_VERSION}.${GLFW_VERSION_PATCH}${GLFW_VERSION_EXTRA}")
-set(LIB_SUFFIX "" CACHE STRING "Takes an empty string or 64. Directory where lib will be installed: lib or lib64")
 
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
@@ -25,6 +24,8 @@ option(GLFW_BUILD_DOCS "Build the GLFW documentation" ON)
 option(GLFW_INSTALL "Generate installation target" ON)
 option(GLFW_VULKAN_STATIC "Use the Vulkan loader statically linked into application" OFF)
 
+include(GNUInstallDirs)
+
 if (UNIX)
     option(GLFW_USE_OSMESA "Use OSMesa for offscreen context creation" OFF)
 endif()
@@ -320,7 +321,7 @@ endforeach()
 #--------------------------------------------------------------------
 include(CMakePackageConfigHelpers)
 
-set(GLFW_CONFIG_PATH "lib${LIB_SUFFIX}/cmake/glfw3")
+set(GLFW_CONFIG_PATH "${CMAKE_INSTALL_LIBDIR}/cmake/glfw3")
 
 configure_package_config_file(src/glfw3Config.cmake.in
                               src/glfw3Config.cmake
@@ -357,7 +358,7 @@ endif()
 # The library is installed by src/CMakeLists.txt
 #--------------------------------------------------------------------
 if (GLFW_INSTALL)
-    install(DIRECTORY include/GLFW DESTINATION include
+    install(DIRECTORY include/GLFW DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
             FILES_MATCHING PATTERN glfw3.h PATTERN glfw3native.h)
 
     install(FILES "${GLFW_BINARY_DIR}/src/glfw3Config.cmake"
@@ -368,7 +369,7 @@ if (GLFW_INSTALL)
             EXPORT_LINK_INTERFACE_LIBRARIES
             DESTINATION "${GLFW_CONFIG_PATH}")
     install(FILES "${GLFW_BINARY_DIR}/src/glfw3.pc"
-            DESTINATION "lib${LIB_SUFFIX}/pkgconfig")
+            DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 
     # Only generate this target if no higher-level project already has
     if (NOT TARGET uninstall)