Fix buildtool failures on non-bash shells and multi-target link/derived-dir handling

[Copilot]

Building MINIXCompat with buildtool on Debian fails in several places: shopt is missing under dash, PROJECT_DERIVED_FILE_DIR is undefined, and a target can't link against another target's static library. This PR fixes those three. It was not built or tested, because GNUstep wasn't available in the authoring environment.

• shopt portability (PBXShellScriptBuildPhase.m, xcsystem.m)

  • The generated scripts now use shopt -s inherit_errexit 2>/dev/null || true. Shells without shopt skip it instead of aborting the script.
  • xcsystem.m uses CRLF line endings, which I preserved.

• PROJECT_DERIVED_FILE_DIR (PBXNativeTarget.m)

  • It is set to derived_src in the build context and the environment. It's set in _productWrapper, alongside TARGET_NAME. Scripts and compile commands that reference it no longer expand it to an empty string, which produced mkdir /Musashi.

• Cross-target linking (PBXFrameworksBuildPhase.m)

  • The link line now adds -L build/<target>/Products for every other target's Products directory, so -lMusashi resolves to libMusashi.a.
  • The current target's own directory is skipped.

-L/usr/local/lib ... -Lbuild/Musashi/Products -Lbuild/m68kmake/Products -lMusashi

Not addressed (from the same issue):

• Target build ordering and the lack of a way to build a single target.
• The Generate 68000 Core script's expectation of a shared Products directory.
• derived_src/Musashi not being on the header search path (m68kops.h).
• buildtool clean leaving build/script_* behind.

• Fixes Handful of issues building MINIXCompat #14