build: fix pkg-config file generation

- Instead of hardcoding "lib" and "include" in libdir and includedir,
  use the values from GNUInstallDirs.

- 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.

- Drop explicit CMAKE_CURRENT_SOURCE_DIR and CMAKE_CURRENT_BINARY_DIR in
  configure_file() as that's implied by default (and quite intuitive).

CMakeLists.txt

@@ -1124,8 +1124,8 @@ 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