Add Ninja build support for precompiled headers

[Copilot]

Adds PCH support to the Ninja build system, matching the existing Makefile implementation.

Changes

• configure.py: Added pch_target variable that evaluates to "pch" when enabled, empty string otherwise. Enables conditional order-only dependencies in Ninja without nested conditionals.

• ninja.in:

  • Added compile_pch_lib and compile_pch_exe rules for compiling separate PCH files for library vs executables/tests
  • Added build targets for pch_lib.h.{gch|pch} and pch_exe.h.{gch|pch}
  • Added phony pch target alias
  • Injected %{pch_include_for_lib} and %{pch_include_for_exe} flags into compilation rules
  • Added order-only dependency | %{pch_target} to all object file builds

When PCH is disabled, the include flags and target variable expand to empty strings, maintaining backward compatibility.

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