Combine `cd` && `sh` in (generated) Makefile

[jonwolski]

Installing this gem failed with the message:

  creating Makefile

  make "DESTDIR=" clean
  rm -rf ./case_transform/target

  make "DESTDIR="
  cd case_transform
  sh ./build.sh
  sh: ./build.sh: No such file or directory
  make: *** [all] Error 127

  make failed, exit code 2

According to https://stackoverflow.com/questions/19985936/current-working-directory-of-makefile#answer-19986164

Each command is executed in its own shell, so "cd" only happens within that shell, but subsequent command is run again from make's current directory.

[NullVoxPopuli]

Transitioning to Helix