Update Meson build system

The following features from the CMake build have been implemented:
 * Library version retrieved from unity.h.
 * Extension support.
 * Library, header, and package configuration file installation.

This commit is entirely based on existing work by Owen Torres.

meson.build

@@ -6,6 +6,17 @@
 #
 project('unity', 'c',
   license: 'MIT',
+
+  # Set project version to value extracted from unity.h header
+  version: run_command(
+    [
+      find_program('python', 'python3'),
+      'auto' / 'extract_version.py',
+      'src' / 'unity.h'
+    ],
+    check: true
+  ).stdout().strip(),
+
   # `meson.project_source_root()` introduced in 0.56.0
   meson_version: '>=0.56.0',
   default_options: [
@@ -14,8 +25,41 @@ project('unity', 'c',
   ]
 )
 
+build_fixture = get_option('extension_fixture').enabled()
+build_memory = get_option('extension_memory').enabled()
+
+unity_src = []
+unity_inc = []
+
 subdir('src')
-unity_dep = declare_dependency(link_with: unity_lib, include_directories: unity_dir)
+
+if build_fixture
+  # Building the fixture extension implies building the memory
+  # extension.
+  build_memory = true
+  subdir('extras/fixture/src')
+endif
+
+if build_memory
+  subdir('extras/memory/src')
+endif
+
+unity_lib = static_library(meson.project_name(),
+  sources: unity_src,
+  include_directories: unity_inc,
+  install: true
+)
+
+unity_dep = declare_dependency(
+  link_with: unity_lib,
+  include_directories: unity_inc
+)
+
+# Generate pkg-config file.
+pkg = import('pkgconfig')
+pkg.generate(unity_lib,
+  description: 'C Unit testing framework.'
+)
 
 # Create a generator that can be used by consumers of our build system to generate
 # test runners.
@@ -24,3 +68,13 @@ gen_test_runner = generator(
   output: '@BASENAME@_Runner.c',
   arguments: ['@INPUT@', '@OUTPUT@']
 )
+
+# Summarize.
+summary(
+  {
+    'Fixture extension': build_fixture,
+    'Memory extension': build_memory,
+  },
+  section: 'Extensions',
+  bool_yn: true
+)