fix: macOS install_name_tool error for monolithic builds

[perazz]

On macOS, if there is no library, fpm install fails for monolithic builds with the error:

  install_name_tool: changing install names or rpaths can't be redone for: ./installed/bin/executable (for architecture arm64) because larger updated load commands do not fit (the
  program must be relinked, and you may need to use -headerpad or -headerpad_max_install_names)

This occurs because:

1. The installer always tries to add rpath entries (@executable_path/../lib and @executable_path) to all executables on macOS
2. Header padding flags (-Wl,-headerpad,0x200) were only applied to executables in non-monolithic builds
3. Monolithic builds (default when no library configuration is specified) produced executables without sufficient header space for install_name_tool to modify

Issue is fixed by always ensuring the necessary header padding also in monolithic executables.

[Copilot]

Pull Request Overview

This PR fixes a macOS installation error for monolithic builds by ensuring proper header padding for all executables. The issue occurred when install_name_tool couldn't modify monolithic executables due to insufficient header space.

• Moves header padding flags from non-monolithic builds only to all executables
• Ensures monolithic builds have adequate header space for install_name_tool modifications

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