chore(CMakeLists.txt): Use GNUinstalldirs module (#327)

GNUInstallDirs module is available since CMake 3.0.2. It provides
standard install directory variables as defined for GNU software.
Using this module would provides standard variable names for
bindir, libdir, includedir and datadir, etc., which would be
beneficial.

This patch modifies the top-level CMakeLists.txt and apply the
standard variables accordingly. The minimal CMake version requirement
is also bumped to 3.0.2.

CMakeLists.txt

@@ -2,13 +2,15 @@ set(CMAKE_USER_MAKE_RULES_OVERRIDE ${CMAKE_CURRENT_SOURCE_DIR}/cmake/c_flag_over
 set(CMAKE_USER_MAKE_RULES_OVERRIDE_CXX ${CMAKE_CURRENT_SOURCE_DIR}/cmake/cxx_flag_overrides.cmake)
 
 project(rime)
-cmake_minimum_required(VERSION 2.8.11)
+cmake_minimum_required(VERSION 3.0.2)
 
 set(rime_version 1.5.3)
 set(rime_soversion 1)
 
 add_definitions(-DRIME_VERSION="${rime_version}")
 
+include(GNUInstallDirs)
+
 option(BUILD_SHARED_LIBS "Build Rime as shared library" ON)
 option(BUILD_MERGED_PLUGINS "Merge plugins into one Rime library" ON)
 option(BUILD_STATIC "Build with dependencies as static libraries" OFF)
@@ -161,11 +163,11 @@ if(UNIX)
 endif()
 
 if(NOT DEFINED LIB_INSTALL_DIR)
-    set(LIB_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/lib${LIB_SUFFIX})
+    set(LIB_INSTALL_DIR ${CMAKE_INSTALL_LIBDIR})
 endif()
 
 if(NOT DEFINED BIN_INSTALL_DIR)
-    set(BIN_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/bin)
+    set(BIN_INSTALL_DIR ${CMAKE_INSTALL_BINDIR})
 endif()
 
 # uninstall target
@@ -179,10 +181,10 @@ add_custom_target(uninstall
 if(${CMAKE_SYSTEM_NAME} MATCHES "Linux|FreeBSD|DragonFly")
   set(prefix "${CMAKE_INSTALL_PREFIX}")
   set(exec_prefix "${CMAKE_INSTALL_PREFIX}")
-  set(bindir "${exec_prefix}/bin")
+  set(bindir "${BIN_INSTALL_DIR}")
   set(libdir "${LIB_INSTALL_DIR}")
   set(pkgdatadir "${prefix}${rime_data_dir}")
-  set(includedir "${CMAKE_INSTALL_PREFIX}/include")
+  set(includedir "${CMAKE_INSTALL_INCLUDEDIR}")
   configure_file(
       ${PROJECT_SOURCE_DIR}/rime.pc.in
       ${PROJECT_BINARY_DIR}/rime.pc