meson: Change the 'tls' option to be boolean.

Change the 'tls' option to be a boolean value instead of a feature.

This still allows manually disabling TLS in builds that would otherwise
support it, but it shouldn't be affected by meson's --auto-features
option.

.gitlab-ci.yml

@@ -84,7 +84,7 @@ build-x86_64-tsd-meson:
   extends:
     - .build-check-meson
   variables:
-    CONFIGURE_OPTIONS: -Dtls=disabled
+    CONFIGURE_OPTIONS: -Dtls=false
 
 build-i386-tsd:
   extends:
@@ -96,7 +96,7 @@ build-i386-tsd-meson:
   extends:
     - .build-check-meson
   variables:
-    CONFIGURE_OPTIONS: -Dtls=disabled --cross-file .gitlab-ci/i686-linux-gnu
+    CONFIGURE_OPTIONS: -Dtls=false --cross-file .gitlab-ci/i686-linux-gnu
 
 build-pure-c-tls:
   extends:
@@ -120,4 +120,4 @@ build-pure-c-tld-meson:
   extends:
     - .build-check-meson
   variables:
-    CONFIGURE_OPTIONS: -Dasm=disabled -Dtls=disabled
+    CONFIGURE_OPTIONS: -Dasm=disabled -Dtls=false

meson.build

@@ -114,14 +114,17 @@ if cc.compiles('typeof(int *);', name : 'typeof')
 endif
 
 with_tls = get_option('tls')
-if with_tls.auto()
+if with_tls
+  # Use a __thread variable if possible, even if we can't use the TLS assembly
+  # stubs, because it's still faster than calling pthread_getspecific.
   have_tls = cc.compiles(
     '__thread int foo;',
     name : '__thread',
   )
 else
-  have_tls = with_tls.enabled()
+  have_tls = false
 endif
+message('Using TLS variable for dispatch table: @0@'.format(have_tls))
 
 if have_tls
   add_project_arguments('-DGLDISPATCH_USE_TLS', language : ['c'])

meson_options.txt

@@ -60,8 +60,9 @@ option(
 )
 option(
   'tls',
-  type : 'feature',
-  description : 'Use Thread Local Storage.'
+  type : 'boolean',
+  value : true,
+  description : 'Use Thread Local Storage (if supported).'
 )
 option(
   'dispatch-page-size',