Add a way to force system asio to be used

CHANGELOG.md

@@ -6,6 +6,23 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic
 Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Added a `system-asio` build option to aid distro packaging.
+
+### Packaging notes
+
+- The new `system-asio` build option forces asio to be used from the standard
+  include directories instead of being defined as a regular Meson dependency.
+  Asio does not have any pkgconfig or CMake [build
+  definitions](https://github.com/chriskohlhoff/asio/issues/1071), so it's
+  impossible to detect its presence and version in a normal way, and as such
+  Meson will fall back to using the included wrap dependency. Configuring the
+  project with `meson setup build -Dsystem-asio=true ...` will cause
+  `<asio.hpp>` to be used instead.
+
 ## [4.0.0] - 2022-06-09
 
 ### Added

meson.build

@@ -22,6 +22,7 @@ project(
 is_64bit_system = build_machine.cpu_family() not in ['x86', 'arm']
 with_32bit_libraries = (not is_64bit_system) or get_option('build.cpp_args').contains('-m32')
 with_bitbridge = get_option('bitbridge')
+with_system_asio = get_option('system-asio')
 with_winedbg = get_option('winedbg')
 with_vst3 = get_option('vst3')
 
@@ -192,7 +193,27 @@ endif
 # These are all headers-only libraries, and thus won't require separate 32-bit
 # and 64-bit versions
 
-asio_dep = dependency('asio', version : '>=1.22.0')
+# NOTE: The standalone asio library does not come with a pkgconfig or CMake
+#       build definition, and Meson thus also won't be able to detect it this
+#       way. As a workaround for distro packaging, configuring the project with
+#       `-Dsystem-asio=true` will use `<asio.h>` from the standard include
+#       directories instead.
+if with_system_asio
+  if not meson.get_compiler('cpp', native : true).check_header('asio.hpp')
+    error('The \'system-asio\' build option was set, but <asio.hpp> was not found')
+  endif
+
+  asio_version = meson.get_compiler('cpp', native : true).get_define('ASIO_VERSION', prefix : '#include <asio.hpp>')
+  if asio_version.to_int() < 102000
+    error('Expected version 1.22.0 of the asio library or higher, found @0@ (MMmmrr)'.format(asio_version))
+  endif
+
+  # This is a dummy dependency, since the library is only accessible implicitly through the system include path
+  asio_dep = declare_dependency()
+else
+  asio_dep = dependency('asio', version : '>=1.22.0')
+endif
+
 if meson.version().version_compare('>=0.60')
   # Bitsery's CMake build definition is capitalized for some reason
   bitsery_dep = dependency('bitsery', 'Bitsery', version : '>=5.2.0')

meson_options.txt

@@ -5,6 +5,18 @@ option(
   description : 'Build a 32-bit host application for hosting 32-bit plugins. See the readme for full instructions on how to use this.'
 )
 
+option(
+  'system-asio',
+  type : 'boolean',
+  value : false,
+  description : '''If set to true, then <asio.hpp> from the standard include
+                   directories will be used in place of a pkgconfig definition,
+                   CMake dependency, or subproject wrap. The asio library does
+                   not come with any build definitions Meson can use to detect
+                   its installed version and location, so this behavior is
+                   behind an option as it's only relevant for distro packaging.'''
+)
+
 option(
   'vst3',
   type : 'boolean',