Generate pkg-config file via CMake (#9945)

Summary:
Fixes https://github.com/facebook/rocksdb/issues/7934

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9945

Test Plan:
Built via Homebrew pointing to my fork/branch:

```
  ~/src/github.com/facebook/fbthrift on   main ❯ cat ~/.homebrew/opt/rocksdb/lib/pkgconfig/rocksdb.pc                                                                                                                                                     took  1h 17m 48s at  04:24:54 pm
prefix="/Users/trynity/.homebrew/Cellar/rocksdb/HEAD-968e4dd"
exec_prefix="${prefix}"
libdir="${prefix}/lib"
includedir="${prefix}/include"

Name: rocksdb
Description: An embeddable persistent key-value store for fast storage
URL: https://rocksdb.org/
Version: 7.3.0
Cflags: -I"${includedir}"
Libs: -L"${libdir}" -lrocksdb
```

Reviewed By: riversand963

Differential Revision: D36161635

Pulled By: trynity

fbshipit-source-id: 0f1a9c30e43797ee65e6696896e06fde0658456e

CMakeLists.txt

@@ -40,6 +40,8 @@ include(GoogleTest)
 get_rocksdb_version(rocksdb_VERSION)
 project(rocksdb
   VERSION ${rocksdb_VERSION}
+  DESCRIPTION "An embeddable persistent key-value store for fast storage"
+  HOMEPAGE_URL https://rocksdb.org/
   LANGUAGES CXX C ASM)
 
 if(POLICY CMP0042)
@@ -1121,6 +1123,12 @@ if(NOT WIN32 OR ROCKSDB_INSTALL_ON_WINDOWS)
     COMPATIBILITY SameMajorVersion
   )
 
+  configure_file(
+    ${CMAKE_CURRENT_SOURCE_DIR}/${PROJECT_NAME}.pc.in
+    ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc
+    @ONLY
+  )
+
   install(DIRECTORY include/rocksdb COMPONENT devel DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
 
   install(DIRECTORY "${PROJECT_SOURCE_DIR}/cmake/modules" COMPONENT devel DESTINATION ${package_config_destination})
@@ -1159,6 +1167,13 @@ if(NOT WIN32 OR ROCKSDB_INSTALL_ON_WINDOWS)
     COMPONENT devel
     DESTINATION ${package_config_destination}
   )
+
+  install(
+    FILES
+    ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc
+    COMPONENT devel
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+  )
 endif()
 
 option(WITH_ALL_TESTS "Build all test, rather than a small subset" ON)

rocksdb.pc.in

@@ -0,0 +1,11 @@
+prefix="@CMAKE_INSTALL_PREFIX@"
+exec_prefix="${prefix}"
+libdir="${prefix}/lib"
+includedir="${prefix}/include"
+
+Name: @PROJECT_NAME@
+Description: @CMAKE_PROJECT_DESCRIPTION@
+URL: @CMAKE_PROJECT_HOMEPAGE_URL@
+Version: @PROJECT_VERSION@
+Cflags: -I"${includedir}"
+Libs: -L"${libdir}" -lrocksdb