Fixes #8687

Motivation / Problem

If path to openttd source contains special characters, CMake may fail to generate the build files.
If build files generation works, build may still fail later.

Description

I solved an issue if path contains some regexp commands (example "Openttd(2++)").
Also solved the issue reported in #8687 (using "OpenTTD - cmake error" as source dir)

Limitations

Compilation fails for "Openttd(2++)" but not for "Openttd (2++)", seems to depend on how ADD_CUSTOM_COMMAND decides to quote commands, and we can't do much about that.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker