Bug 1390524 - correctly pass extra_toolchain_flags only to the target; r=glandium

We define extra_toolchain_flags for passing extra flags to the target
compiler during configure.  But the way things are currently set up, we
pass those flags to the host compiler during configure as well.  This
behavior is incorrect, and we should only be passing the flags from
extra_toolchain_flags if we're compiling for the target.

build/moz.configure/compilers-util.configure

@@ -7,7 +7,9 @@
 @template
 @imports('textwrap')
 @imports(_from='mozbuild.configure', _import='SandboxDependsFunction')
-def compiler_class(compiler):
+def compiler_class(compiler, host_or_target):
+    is_target = host_or_target is target
+
     class Compiler(SandboxDependsFunction):
         # Generates a test program and attempts to compile it. In case of
         # failure, the resulting check will return None. If the test program
@@ -47,7 +49,8 @@ def compiler_class(compiler):
             @checking_fn
             def func(compiler, flags, extra_flags):
                 flags = flags or []
-                flags += extra_flags or []
+                if is_target:
+                    flags += extra_flags or []
                 flags.append('-c')
 
                 if try_invoke_compiler(

build/moz.configure/toolchain.configure

@@ -809,7 +809,7 @@ def compiler(language, host_or_target, c_compiler=None, other_compiler=None,
         add_old_configure_assignment(
             '%s_VERSION' % var, valid_compiler.version)
 
-    valid_compiler = compiler_class(valid_compiler)
+    valid_compiler = compiler_class(valid_compiler, host_or_target)
 
     def compiler_error():
         raise FatalCheckError('Failed compiling a simple %s source with %s'