feat: cpp preprocessor macros: simple file parsing

[perazz]

With the introduction of features and profiles, it will be nearly impossible to avoid parsing preprocessor macro-guarded
module inclusions. This PR introduces the conditional block detection logic to properly handle nested simple conditional blocks and (case-sensitive) macro matching with the cpp preprocessor.

• Supported syntax examples

  #ifdef FEATURE_DEBUG
      use debug_module
  #endif

  #ifndef FEATURE_RELEASE
      use development_module
  #endif

  #if defined(MY_FEATURE)
      use feature_module
  #endif

  #if !defined(OTHER_FEATURE)
      use fallback_module
      #ifdef NESTED_FEATURE
          use nested_module
      #endif
  #endif

Tests and example packages added. Closes #469.

[Copilot]

Pull Request Overview

This PR introduces support for parsing C preprocessor (CPP) conditional compilation directives in Fortran source files. This enables proper handling of macro-guarded module dependencies when features and profiles are used, preventing build failures when optional modules are conditionally included.

Key changes include:

• Addition of conditional compilation parsing logic to handle nested #ifdef, #ifndef, #if defined(), and related directives
• Integration of preprocessor configuration into source parsing routines throughout the codebase
• Comprehensive test coverage for various conditional compilation scenarios

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/fpm_test/test_source_parsing.f90	Adds comprehensive test cases for conditional compilation parsing with both simple and complex macro scenarios
src/fpm_source_parsing.f90	Implements core CPP conditional parsing logic with nested block tracking and macro evaluation
src/fpm_sources.f90	Updates source discovery functions to accept and pass through preprocessor configuration
src/fpm.f90	Integrates preprocessor configuration into all source parsing calls throughout the build system
example_packages/program_with_cpp_guarded_module/	Provides example package demonstrating CPP conditional compilation usage
ci/run_tests.sh	Adds testing of the new example package to the CI pipeline

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