Quad double: Add initial/tentative support for libqd.

config/Jamfile.v2

@@ -44,6 +44,7 @@ lib mpc ;
 lib quadmath ;
 lib tommath ;
 lib f2c ;
+lib qd ;
 
 actions mp_simple_run_action
 {
@@ -61,6 +62,7 @@ rule mp-run-simple ( sources + : args * : input-files * : requirements * : targe
 
 mp-run-simple has_gmp.cpp gmp : : :
       <include>$(gmp_path) <include>$(gmp_path)/mpfr <include>$(gmp_path)/gmpfrxx : has_gmp ;
+mp-run-simple has_qd.cpp qd : : : : has_qd ;
 mp-run-simple has_mpfr.cpp mpfr gmp : : :
       <include>$(mpfr_path) <include>$(gmp_path)/mpfr <include>$(gmp_path)/gmpfrxx <include>$(gmp_path) : has_mpfr ;
 mp-run-simple has_mpfi.cpp mpfi mpfr gmp : : :
@@ -78,6 +80,7 @@ obj has_constexpr_limits : has_constexpr_limits_cmd.cpp : <cxxflags>-fconstexpr-
 obj has_big_obj : has_big_obj.cpp : <cxxflags>-Wa,-mbig-obj ;
 
 explicit has_gmp ;
+explicit has_qd ;
 explicit has_mpfr ;
 explicit has_mpfi ;
 explicit has_tommath ;

config/has_qd.cpp

@@ -0,0 +1,12 @@
+//  Copyright John Maddock 2020.
+//  Use, modification and distribution are subject to the
+//  Boost Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <qd/qd_real.h>
+
+int main()
+{
+   qd_real q;
+   return 0;
+}