Adds a CMake option to build `libprotoc` separately (#7949)

This exposes the variable `protobuf_BUILD_LIBPROTOC` as CMake option to be able to build libprotoc without building the protoc binary.

cmake/CMakeLists.txt

@@ -44,6 +44,7 @@ option(protobuf_BUILD_TESTS "Build tests" ON)
 option(protobuf_BUILD_CONFORMANCE "Build conformance tests" OFF)
 option(protobuf_BUILD_EXAMPLES "Build examples" OFF)
 option(protobuf_BUILD_PROTOC_BINARIES "Build libprotoc and protoc compiler" ON)
+option(protobuf_BUILD_LIBPROTOC "Build libprotoc" OFF)
 if (BUILD_SHARED_LIBS)
   set(protobuf_BUILD_SHARED_LIBS_DEFAULT ON)
 else (BUILD_SHARED_LIBS)
@@ -64,8 +65,6 @@ include(protobuf-options.cmake)
 # Overrides for option dependencies
 if (protobuf_BUILD_PROTOC_BINARIES OR protobuf_BUILD_TESTS)
   set(protobuf_BUILD_LIBPROTOC ON)
-else()
-  set(protobuf_BUILD_LIBPROTOC OFF)
 endif ()
 # Path to main configure script
 set(protobuf_CONFIGURE_SCRIPT "../configure.ac")

cmake/install.cmake

@@ -6,9 +6,9 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/protobuf-lite.pc.cmake
                ${CMAKE_CURRENT_BINARY_DIR}/protobuf-lite.pc @ONLY)
 
 set(_protobuf_libraries libprotobuf-lite libprotobuf)
-if (protobuf_BUILD_PROTOC_BINARIES)
+if (protobuf_BUILD_LIBPROTOC)
     list(APPEND _protobuf_libraries libprotoc)
-endif (protobuf_BUILD_PROTOC_BINARIES)
+endif (protobuf_BUILD_LIBPROTOC)
 
 foreach(_library ${_protobuf_libraries})
   set_property(TARGET ${_library}