meson: use a feature option for libunwind

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20409>

meson.build

@@ -1901,26 +1901,16 @@ if get_option('execmem')
   pre_args += '-DMESA_EXECMEM'
 endif
 
-_libunwind = get_option('libunwind')
+_libunwind = get_option('libunwind') \
-if _libunwind == 'true'
+  .require(not with_platform_android, error_message : 'Android requires the use of the backtrace library, not libunwind')
-  _libunwind = 'enabled'
-  warning('libunwind option "true" deprecated, please use "enabled" instead.')
-elif _libunwind == 'false'
-  _libunwind = 'disabled'
-  warning('libunwind option "false" deprecated, please use "disabled" instead.')
-endif
-if _libunwind != 'disabled' and not with_platform_android
 if host_machine.system() == 'darwin'
-    dep_unwind = meson.get_compiler('c').find_library('System')
+  dep_unwind = meson.get_compiler('c').find_library('System', required : _libunwind)
 else
-    dep_unwind = dependency('libunwind', required : _libunwind == 'enabled')
+  dep_unwind = dependency('libunwind', required : _libunwind)
 endif
 if dep_unwind.found()
   pre_args += '-DHAVE_LIBUNWIND'
 endif
-else
-  dep_unwind = null_dep
-endif
 
 if with_osmesa
   if not with_gallium_softpipe

meson_options.txt

@@ -340,9 +340,7 @@ option(
 )
 option(
   'libunwind',
-  type : 'combo',
+  type : 'feature',
-  value : 'auto',
-  choices : ['auto', 'true', 'false', 'enabled', 'disabled'],
   description : 'Use libunwind for stack-traces'
 )
 option(