Updates to Meson build system:

1. Use cross-platform `/` operator for path construction.

2. Use `meson.project_source_root()` for correct path resolution of
   generate_test_runner.rb path when used as a subproject.

3. Bump the minimum required Meson version to '0.56.0' as this is
   needed for the above changes.

meson.build

@@ -5,21 +5,22 @@
 # license: MIT
 #
 project('unity', 'c',
-    license: 'MIT',
-    meson_version: '>=0.37.0',
-    default_options: ['werror=true', 'c_std=c11'])
+  license: 'MIT',
+  # `meson.project_source_root()` introduced in 0.56.0
+  meson_version: '>=0.56.0',
+  default_options: [
+    'werror=true',
+    'c_std=c11'
+  ]
+)
 
 subdir('src')
 unity_dep = declare_dependency(link_with: unity_lib, include_directories: unity_dir)
 
-
-# Get the generate_test_runner script relative to itself or the parent project if it is being used as a subproject
-# NOTE: This could be (and probably is) a complete hack - but I haven't yet been able to find a better way....
-if meson.is_subproject()
-gen_test_runner_path = find_program(meson.source_root() / 'subprojects/unity/auto/generate_test_runner.rb')
-else
-gen_test_runner_path = find_program('subprojects/unity/auto/generate_test_runner.rb')
-endif
-
-# Create a generator that we can access from the parent project
-gen_test_runner = generator(gen_test_runner_path, output: '@BASENAME@_Runner.c', arguments: ['@INPUT@', '@OUTPUT@'] )
+# Create a generator that can be used by consumers of our build system to generate
+# test runners.
+gen_test_runner = generator(
+  find_program(meson.project_source_root() / 'auto' / 'generate_test_runner.rb'),
+  output: '@BASENAME@_Runner.c',
+  arguments: ['@INPUT@', '@OUTPUT@']
+)