Even if presym remains unchanged, updating a C file triggers a recompilation of the entire build target #6721

Even if presym remains unchanged, updating any C file in mrbgems triggers a recompilation of mruby core's C files.

• mruby revision: commit 2031ae9
• build configuration: default

• % rake
  ...omit...
  % touch mrbgems/mruby-struct/src/struct.c
  % rake
  mruby-io: No HAL specified, loading hal-posix-io (explicit selection recommended)
  mruby-socket: No HAL specified, loading hal-posix-socket (explicit selection recommended)
  mruby-dir: No HAL specified, loading hal-posix-dir (explicit selection recommended)
  CPP   mrbgems/mruby-struct/src/struct.c -> build/host/mrbgems/mruby-struct/src/struct.pi
  CC    mrbgems/mruby-bigint/core/bigint.c -> build/host/mrbgems/mruby-bigint/core/bigint.o
  CC    src/allocf.c -> build/host/src/allocf.o
  CC    src/array.c -> build/host/src/array.o
  CC    src/backtrace.c -> build/host/src/backtrace.o
  CC    src/cdump.c -> build/host/src/cdump.o
  CC    src/class.c -> build/host/src/class.o
  CC    src/codedump.c -> build/host/src/codedump.o
  CC    src/debug.c -> build/host/src/debug.o
  CC    src/dump.c -> build/host/src/dump.o
  CC    src/enum.c -> build/host/src/enum.o
  CC    src/error.c -> build/host/src/error.o
  CC    src/etc.c -> build/host/src/etc.o
  ...omit...

At commit cf52364, only tasks dependent on mrbgems/mruby-struct/src/struct.c were triggered.