meson: Fix boost static linking logic

Do not attempt to statically link boost for -Ddefault_library=static:
-Dprefer_static=true exists now and should be used to statically link
external dependencies, while -Ddefault_library=static is meant for
internal dependencies.

However, if -Dprefer_static=true *is* passed (I'm not aware of any setup
where it is, but it doesn't hurt to be correct here), we need to work
around two meson bugs here.

Author: arch1t3cht

meson.build

@@ -106,20 +106,26 @@ deps += dependency('iconv', fallback: ['iconv', 'libiconv_dep'])
 deps += dependency('libass', version: '>=0.9.7',
                    fallback: ['libass', 'libass_dep'])
 
+boost_static = get_option('prefer_static')    # boost dependencies do not respect prefer_static https://github.com/mesonbuild/meson/issues/15458
 boost_modules = ['chrono', 'locale']
+
+if boost_static
+    boost_modules += 'thread'   # static boost dependencies do not include transitive dependencies https://github.com/mesonbuild/meson/issues/15457
+endif
+
 if not get_option('local_boost')
     boost_dep = dependency('boost', version: '>=1.70.0',
                             modules: boost_modules,
                             required: false,
-                            static: get_option('default_library') == 'static')
+                            static: boost_static)
 
     if boost_dep.version().version_compare('<1.77.0')
         # boost regex is header-only since version 1.77.0, and some distributions have stopped shipping the stub module since then.
         # So we need to require boost.regex for older versions but can't require it for newer versions.
         boost_dep = dependency('boost', version: '>=1.70.0',
                                 modules: boost_modules + ['regex'],
                                 required: false,
-                                static: get_option('default_library') == 'static')
+                                static: boost_static)
     endif
 endif