Add plumb meson version into ashuffle.

We use Meson's `configure_file` to fill in a string variable in a new
`libversion` library. This value is printed when a version flag is
provided.

meson.build

@@ -1,5 +1,10 @@
 # vim: set sw=2 ts=2:
-project('ashuffle', ['c', 'cpp'], default_options: ['cpp_std=c++17', 'warning_level=2'])
+project(
+  'ashuffle',
+  ['c', 'cpp'],
+  version: 'v3.10.1',
+  default_options: ['cpp_std=c++17', 'warning_level=2']
+)
 
 add_global_arguments(
   [
@@ -71,6 +76,22 @@ else
 endif
 libmpdclient = dependency('libmpdclient')
 
+src_inc = include_directories('src')
+
+version_cc = configure_file(
+  configuration: {
+    'VERSION': meson.project_version(),
+  },
+  input: 'src/version.cc.in',
+  output: 'version.cc',
+)
+
+libversion = static_library(
+  'version',
+  [version_cc],
+  include_directories: src_inc,
+)
+
 sources = files(
   'src/ashuffle.cc',
   'src/load.cc',
@@ -82,14 +103,12 @@ sources = files(
 
 executable_sources = sources + files('src/mpd_client.cc', 'src/main.cc')
 
-src_inc = include_directories('src')
-root_inc = include_directories('.')
-
 ashuffle = executable(
   'ashuffle',
   executable_sources,
   dependencies: absl_deps + [libmpdclient],
-  install: true
+  link_with: libversion,
+  install: true,
 )
 
 clang_tidy = run_target('ashuffle-clang-tidy',

src/main.cc

@@ -15,6 +15,7 @@
 #include "load.h"
 #include "mpd_client.h"
 #include "shuffle.h"
+#include "version.h"
 
 using namespace ashuffle;
 
@@ -41,8 +42,8 @@ int main(int argc, const char* argv[]) {
             case ParseError::Type::kVersion:
                 // Don't print help in this case, since the user specifically
                 // requested we print the version.
-                std::cout << "version: Unknown" << std::endl;
-                exit(EXIT_FAILURE);
+                std::cout << "ashuffle version: " << kVersion << std::endl;
+                exit(EXIT_SUCCESS);
             case ParseError::Type::kUnknown:
                 std::cerr << "unknown option parsing error. Please file a bug "
                           << "at https://github.com/joshkunz/ashuffle"

src/version.cc.in

@@ -0,0 +1,8 @@
+#include "version.h"
+
+namespace ashuffle {
+
+// Filled in by Meson.
+const char* kVersion = "@VERSION@";
+
+}  // namespace ashuffle

src/version.h

@@ -0,0 +1,13 @@
+
+#ifndef __ASHUFFLE_VERSION_H__
+#define __ASHUFFLE_VERSION_H__
+
+namespace ashuffle {
+
+// kVersion is filled in by the build system to contain the version of
+// ashuffle.
+extern const char* kVersion;
+
+}  // namespace ashuffle
+
+#endif  // __ASHUFFLE_VERSION_H__