utils/.../ninja.py: Support CXX for ninja builds on Linux.

Without this fix, on Linux, passing --build-ninja looks for g++
and fails to build on the CI.

Author: Doug Coleman

utils/swift_build_support/swift_build_support/products/ninja.py

@@ -47,6 +47,10 @@ def do_build(self):
                     "-mmacosx-version-min={osx_version}"
                 ).format(osx_version=osx_version_min),
             }
+        elif self.toolchain.cxx:
+            env = {
+                "CXX": self.toolchain.cxx,
+            }
 
         # Ninja can only be built in-tree.  Copy the source tree to the build
         # directory.

utils/swift_build_support/tests/products/test_ninja.py

@@ -99,6 +99,13 @@ def test_do_build(self):
                 cxx=self.toolchain.cxx,
                 sysroot=xcrun.sdk_path('macosx')
             )
+        elif self.toolchain.cxx:
+            expect_env = (
+                "env "
+                "CXX={cxx} "
+            ).format(
+                cxx=self.toolchain.cxx,
+            )
 
         self.assertEqual(self.stdout.getvalue(), """\
 + rm -rf {build_dir}