Auto-detection of <threadapi> default value

Jamroot

@@ -129,6 +129,7 @@ import xsltproc ;
 import set ;
 import path ;
 import link ;
+import threadapi-feature ;
 
 path-constant BOOST_ROOT : . ;
 constant BOOST_VERSION : 1.66.0 ;
@@ -169,6 +170,7 @@ project boost
       <toolset>como-linux:<define>_GNU_SOURCE=1
       # When building docs within Boost, we want the standard Boost style
       <xsl:param>boost.defaults=Boost
+      <conditional>@threadapi-feature.detect
     : usage-requirements <include>.
     : build-dir bin.v2
     ;