build: fix pkg-config file generation (facebook#9953)

Summary:
- Instead of hardcoding "lib" and "include" in `libdir` and `includedir`, use the values from [`GNUInstallDirs`](https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html).

- Use `PROJECT_DESCRIPTION` and `PROJECT_HOMEPAGE_URL` instead of their
`CMAKE_` conterparts to fix pkg-config generation when rocksdb is not the top-level project (see [`project()`](https://cmake.org/cmake/help/latest/command/project.html)).

- Drop explicit `CMAKE_CURRENT_SOURCE_DIR` and `CMAKE_CURRENT_BINARY_DIR` in [`configure_file()`](https://cmake.org/cmake/help/latest/command/configure_file.html) as that's implied by default (and quite intuitive).

See facebook#9945
CC: trynity

Pull Request resolved: facebook#9953

Reviewed By: ajkr

Differential Revision: D36716373

Pulled By: jay-zhuang

fbshipit-source-id: 57840eeb4453099fa1fe861dc03366085dbca704

CMakeLists.txt

@@ -1122,8 +1122,8 @@ if(NOT WIN32 OR ROCKSDB_INSTALL_ON_WINDOWS)
   )
 
   configure_file(
-    ${CMAKE_CURRENT_SOURCE_DIR}/${PROJECT_NAME}.pc.in
-    ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc
+    ${PROJECT_NAME}.pc.in
+    ${PROJECT_NAME}.pc
     @ONLY
   )
 

rocksdb.pc.in

@@ -1,11 +1,10 @@
 prefix="@CMAKE_INSTALL_PREFIX@"
-exec_prefix="${prefix}"
-libdir="${prefix}/lib"
-includedir="${prefix}/include"
+includedir="${prefix}/@CMAKE_INSTALL_INCLUDEDIR@"
+libdir="${prefix}/@CMAKE_INSTALL_LIBDIR@"
 
 Name: @PROJECT_NAME@
-Description: @CMAKE_PROJECT_DESCRIPTION@
-URL: @CMAKE_PROJECT_HOMEPAGE_URL@
+Description: @PROJECT_DESCRIPTION@
+URL: @PROJECT_HOMEPAGE_URL@
 Version: @PROJECT_VERSION@
 Cflags: -I"${includedir}"
 Libs: -L"${libdir}" -lrocksdb