Don't build numpy if there's no python version to target.

build/Jamfile

@@ -42,8 +42,11 @@ rule cond ( test ? : yes * : no * ) { if $(test) { return $(yes) ; } else { retu
 rule unless ( test ? : yes * : no * ) { if ! $(test) { return $(yes) ; } else { return $(no) ; } }
 local rule eq ( a : b ) { if $(a) = $(b) { return 1 ; } }
 
-lib_boost_python($(py2-version)) = boost_python ;
-lib_boost_python($(py3-version)) = boost_python3 ;
+lib_boost_python(2) = boost_python ;
+lib_boost_python(3) = boost_python3 ;
+
+lib_boost_python($(py2-version)) = $(lib_boost_python(2)) ;
+lib_boost_python($(py3-version)) = $(lib_boost_python(3)) ;
 
 rule lib_boost_python ( version )
 {
@@ -118,8 +121,11 @@ rule lib_boost_python ( version )
         ;
 }
 
-lib_boost_numpy($(py2-version)) = boost_numpy ;
-lib_boost_numpy($(py3-version)) = boost_numpy3 ;
+lib_boost_numpy(2) = boost_numpy ;
+lib_boost_numpy(3) = boost_numpy3 ;
+
+lib_boost_numpy($(py2-version)) = $(lib_boost_python(2)) ;
+lib_boost_numpy($(py3-version)) = $(lib_boost_python(3)) ;
 
 rule lib_boost_numpy ( version )
 {
@@ -163,11 +169,19 @@ for local N in 2 3
     {
         lib_boost_python $(py$(N)-version) ;
         libraries += $(lib_boost_python($(py$(N)-version))) ;
-        if [ python.numpy ]
-        {
-            lib_boost_numpy $(py$(N)-version) ;
-            libraries += $(lib_boost_numpy($(py$(N)-version))) ;
-        }
+    }
+    else
+    {
+        alias $(lib_boost_python($(N))) ;
+    }
+    if $(py$(N)-version) && [ python.numpy ]
+    {
+        lib_boost_numpy $(py$(N)-version) ;
+        libraries += $(lib_boost_numpy($(py$(N)-version))) ;
+    }
+    else
+    {
+        alias $(lib_boost_numpy($(N))) ;
     }
 }