CMake: Install .pc Files

Adds pkg-config (`.pc`) files for CMake based installs.

cmake/install.cmake

 include(GNUInstallDirs)
 
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/protobuf.pc.cmake
+               ${CMAKE_CURRENT_BINARY_DIR}/protobuf.pc @ONLY)
+configure_file(${CMAKE_CURRENT_SOURCE_DIR}/protobuf-lite.pc.cmake
+               ${CMAKE_CURRENT_BINARY_DIR}/protobuf-lite.pc @ONLY)
+
 foreach(_library
   libprotobuf-lite
   libprotobuf
@@ -17,6 +22,8 @@ endforeach()
 install(TARGETS protoc EXPORT protobuf-targets
   RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT protoc)
 
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/protobuf.pc ${CMAKE_CURRENT_BINARY_DIR}/protobuf-lite.pc DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+
 file(STRINGS extract_includes.bat.in _extract_strings
   REGEX "^copy")
 foreach(_extract_string ${_extract_strings})

cmake/protobuf-lite.pc.cmake

+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=@CMAKE_INSTALL_PREFIX@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+
+Name: Protocol Buffers
+Description: Google's Data Interchange Format
+Version: @protobuf_VERSION@
+Libs: -L${libdir} -lprotobuf-lite @CMAKE_THREAD_LIBS_INIT@
+Cflags: -I${includedir} @CMAKE_THREAD_LIBS_INIT@
+Conflicts: protobuf

cmake/protobuf.pc.cmake

+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=@CMAKE_INSTALL_PREFIX@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
+
+Name: Protocol Buffers
+Description: Google's Data Interchange Format
+Version: @protobuf_VERSION@
+Libs: -L${libdir} -lprotobuf @CMAKE_THREAD_LIBS_INIT@
+Cflags: -I${includedir} @CMAKE_THREAD_LIBS_INIT@
+Conflicts: protobuf-lite