allow explicit target triple for clang-linux and clang-darwin

src/tools/clang-darwin.jam

@@ -50,7 +50,8 @@ rule init ( version ? :  command * : options * )
         : version $(version) ] ;
 
     common.handle-options clang-darwin : $(condition) : $(command) : $(options) ;
-    clang.init-flags clang-darwin : $(condition) : $(version) ;
+    clang.init-flags clang-darwin : $(condition) : $(version) :
+        [ feature.get-values <triplet> : $(options) ] ;
 
     # - Archive builder.
     local archiver = [ feature.get-values <archiver> : $(options) ] ;

src/tools/clang-linux.jam

@@ -13,7 +13,13 @@
 = Clang (GCC frontend)
 
 The `clang-linux` module supports Clang with GCC frontend and has the same
-options as link:#b2.reference.tools.compiler.gcc[`gcc`] toolset.
+options as link:#b2.reference.tools.compiler.gcc[`gcc`] toolset, with the
+addition of `<triple>` option.
+
+The value of that option is passed as the value of `--target=` flag, unless
+provided as `<triple>none`. In that case the flag is not passed to the build
+tools. If the option is not provided the build system may try to guess the
+correct target triple.
 
 |# # end::doc[]
 
@@ -68,7 +74,8 @@ rule init ( version ? :  command * : options * ) {
     : version $(version) ] ;
 
   common.handle-options clang-linux : $(condition) : $(command) : $(options) ;
-  clang.init-flags clang-linux : $(condition) : $(version) ;
+  clang.init-flags clang-linux : $(condition) : $(version) :
+      [ feature.get-values <triple> : $(options) ] ;
 
   # Support for gcc root as the backend, this is mainly useful for clang/gcc on Windows
   # since on Linux gcc will be the default compiler already located on the PATH.

src/tools/clang.jam

@@ -92,20 +92,36 @@ local rule init-flags-cross ( toolset : condition * : architecture + : address-m
                 case x86-32 : arch = i386 ;
             }
 
-            toolset.flags $(toolset)
-                OPTIONS $(condition)/<target-os>$(target-os)/<architecture>$(_architecture_)/<address-model>$(_address-model_)
-                : "--target=$(arch)-$(vendor-sys)"
-                : unchecked ;
+            set-triple $(toolset) :
+                $(condition)/<target-os>$(target-os)/<architecture>$(_architecture_)/<address-model>$(_address-model_)
+                : "$(arch)-$(vendor-sys)"
+                ;
         }
     }
 }
 
-rule init-flags ( toolset : condition * : version )
+local rule set-triple ( toolset : condition  : triple )
+{
+    toolset.flags $(toolset) OPTIONS $(condition) : --target=$(triple)
+        : unchecked ;
+}
+
+rule init-flags ( toolset : condition * : version : triple ? )
 {
     init-cxxstd-flags $(toolset) : $(condition) : $(version) ;
 
-    init-flags-cross $(toolset) : $(condition) : arm x86 : 64 : darwin ;
-    init-flags-cross $(toolset) : $(condition) : arm x86 : 64 32 : linux ;
+    if ! $(triple)
+    {
+        init-flags-cross $(toolset) : $(condition) : arm x86 : 64 : darwin ;
+        init-flags-cross $(toolset) : $(condition) : arm x86 : 64 32 : linux ;
+    }
+    else
+    {
+        if $(triple) != none
+        {
+            set-triple $(toolset) : $(condition) : $(triple) ;
+        }
+    }
 
     # This is a temporary solution for doing combined architecture builds on macOS.
     toolset.flags $(toolset)