[CMake] generate pkg-config flatbuffers.pc file (#6455)

This commit enables CMake to generate a flatbuffers.pc file on install.

pkg-config eases the utilization of software libraries by enabling the
developers of a library to define how the library should be used.

It can be used as a tool by build systems to find and manage dependencies,
this is notably the default Meson behavior and optionally used by CMake.

CMakeLists.txt

@@ -629,6 +629,14 @@ if(FLATBUFFERS_INSTALL)
       DESTINATION ${FB_CMAKE_DIR}
     )
   endif()
+
+  if(FLATBUFFERS_BUILD_SHAREDLIB OR FLATBUFFERS_BUILD_FLATLIB)
+      configure_file(CMake/flatbuffers.pc.in flatbuffers.pc @ONLY)
+      install(
+        FILES "${CMAKE_CURRENT_BINARY_DIR}/flatbuffers.pc"
+        DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+      )
+  endif()
 endif()
 
 if(FLATBUFFERS_BUILD_TESTS)