Meson: Add 'coretracers' feature option

This was the only plugin still built when using
-Dauto_features=disabled, besides coreelements.

meson_options.txt

@@ -23,6 +23,7 @@ option('libunwind', type : 'feature', value : 'auto', description : 'Use libunwi
 option('libdw', type : 'feature', value : 'auto', description : 'Use libdw to generate better backtraces from libunwind')
 option('dbghelp', type : 'feature', value : 'auto', description : 'Use dbghelp to generate backtraces')
 option('bash-completion', type : 'feature', value : 'auto', description : 'Install bash completion files')
+option('coretracers', type : 'feature', value : 'auto', description : 'Build coretracers plugin')
 
 # Common feature options
 option('examples', type : 'feature', value : 'auto', yield : true)

plugins/meson.build

@@ -1,5 +1,5 @@
 plugins = []
 subdir('elements')
-if tracer_hooks
+if not get_option('coretracers').disabled()
   subdir('tracers')
 endif

plugins/tracers/meson.build

@@ -1,3 +1,10 @@
+if not tracer_hooks
+  if get_option('coretracers').enabled()
+    error('coretracers plugin enabled but not tracer_hooks')
+  endif
+  subdir_done()
+endif
+
 gst_tracers_sources = [
   'gstlatency.c',
   'gstleaks.c',