Allow using system absl

Most distros don't like projects embedding subprojects, so
we need to be capable of building using the system's absl.
It is not going to work easily because the system's absl
needs to be built with the C++17 standard to work, so
ultimately this might not be such a good idea, but it
works for me on nixos...

meson.build

@@ -1,34 +1,51 @@
 project('ashuffle', ['c', 'cpp'], default_options: ['cpp_std=c++17', 'warning_level=2'])
 
 cmake = import('cmake')
+cpp = meson.get_compiler('cpp')
 
-absl = cmake.subproject('absl', cmake_options: [
-  '-DCMAKE_CXX_STANDARD=17',
-])
-
-# Note: Unfortunately, meson does not pull in the "dependent" libraries
-# correctly so we have to list them here. If you take a dependency on some
-# addtional absl features, you will probably need to add more `dependency`
-# lines here. This could also be broken on upgrades.
-absl_all = [
+# absl dependencies need to be explicited...
+# It might be possible to use cmake dependencies (e.g. "absl:string"
+# defined in abslTargets.cmake in the future but that does not seem
+# worth the time trying to figure that out.
+absl_libs = [
     # Via Base:
-    absl.dependency('raw_logging_internal'),
+    'raw_logging_internal',
 
     # Via Strings:
-    absl.dependency('int128'),
-    absl.dependency('str_format_internal'),
-    absl.dependency('strings_internal'),
-    absl.dependency('strings'),
+    'int128',
+    'str_format_internal',
+    'strings_internal',
+    'strings'
 ]
 
+absl_deps = []
+if not get_option('unsupported_use_system_absl')
+    absl = cmake.subproject('absl', cmake_options: [
+        '-DCMAKE_CXX_STANDARD=17',
+    ])
+
+    absl_deps = []
+    foreach lib : absl_libs
+        absl_deps += absl.dependency(lib)
+    endforeach
+else
+    # note that the system's absl needs to be compiled for C++17 standard
+    # or final link will fail.
+    foreach lib : absl_libs
+        dep = cpp.find_library('absl_' + lib)
+        if dep.found()
+            absl_deps += dep
+        endif
+    endforeach
+endif
 libmpdclient = dependency('libmpdclient')
 
 sources = files(
     'src/ashuffle.cc',
     'src/load.cc',
-	'src/args.cc',
-	'src/getpass.cc',
-	'src/rule.cc',
+    'src/args.cc',
+    'src/getpass.cc',
+    'src/rule.cc',
     'src/shuffle.cc',
 )
 
@@ -38,55 +55,54 @@ src_inc = include_directories('src')
 root_inc = include_directories('.')
 
 ashuffle = executable(
-	'ashuffle',
-	executable_sources,
-	dependencies: absl_all + [libmpdclient],
-	install: true
+    'ashuffle',
+    executable_sources,
+    dependencies: absl_deps + [libmpdclient],
+    install: true
 )
 
 clang_tidy = run_target('ashuffle-clang-tidy',
     command : files('scripts/run-clang-tidy') + executable_sources
 )
 
-# We only generate tests if we are *not running in tidy mode
 if get_option('tests').enabled()
 
-googletest = cmake.subproject('googletest', cmake_options: [
-  '-DBUILD_GMOCK=ON',
-  '-DCMAKE_CXX_STANDARD=17',
-])
-
-gtest_deps = [
-  dependency('threads'),
-  googletest.dependency('gtest'),
-  googletest.dependency('gmock'),
-  googletest.dependency('gmock_main'),
-]
-
-mpdfake_inc = include_directories('t')
-mpdfake_dep = declare_dependency(include_directories : mpdfake_inc)
-
-test_options = [
-    'werror=true',
-]
-
-tests = {
-    'rule': ['t/rule_test.cc'],
-    'shuffle': ['t/shuffle_test.cc'],
-    'load': ['t/load_test.cc'],
-    'args': ['t/args_test.cc'],
-    'ashuffle': ['t/ashuffle_test.cc'],
-}
-
-foreach test_name, test_sources : tests
-    test_exe = executable(
-        test_name + '_test',
-        sources + test_sources,
-        include_directories : src_inc,
-        dependencies : absl_all + gtest_deps + [mpdfake_dep],
-        override_options : test_options,
-    )
-    test(test_name, test_exe)
-endforeach
+    googletest = cmake.subproject('googletest', cmake_options: [
+        '-DBUILD_GMOCK=ON',
+        '-DCMAKE_CXX_STANDARD=17',
+    ])
+
+    gtest_deps = [
+        dependency('threads'),
+        googletest.dependency('gtest'),
+        googletest.dependency('gmock'),
+        googletest.dependency('gmock_main'),
+    ]
+
+    mpdfake_inc = include_directories('t')
+    mpdfake_dep = declare_dependency(include_directories : mpdfake_inc)
+
+    test_options = [
+        'werror=true',
+    ]
+
+    tests = {
+        'rule': ['t/rule_test.cc'],
+        'shuffle': ['t/shuffle_test.cc'],
+        'load': ['t/load_test.cc'],
+        'args': ['t/args_test.cc'],
+        'ashuffle': ['t/ashuffle_test.cc'],
+    }
+
+    foreach test_name, test_sources : tests
+        test_exe = executable(
+            test_name + '_test',
+            sources + test_sources,
+            include_directories : src_inc,
+            dependencies : absl_deps, gtest_deps + [mpdfake_dep],
+            override_options : test_options,
+        )
+        test(test_name, test_exe)
+    endforeach
 
 endif # tests feature

meson_options.txt

@@ -1,2 +1,2 @@
-option('tidy_mode', type : 'feature', value : 'disabled')
 option('tests', type : 'feature', value : 'disabled')
+option('unsupported_use_system_absl', type : 'boolean', value : 'false')