ENH: Test transitive link languages

This test creates a C executable that links to a C++ static library.  On
most platforms the executable will not link unless the C++ linker is
chosen correctly.

Tests/CMakeLists.txt

@@ -92,6 +92,7 @@ IF(BUILD_TESTING)
   ADD_TEST_MACRO(SetLang SetLang)
   ADD_TEST_MACRO(ExternalOBJ ExternalOBJ)
   ADD_TEST_MACRO(LoadCommand LoadedCommand)
+  ADD_TEST_MACRO(LinkLanguage LinkLanguage)
   ADD_TEST_MACRO(LinkLine LinkLine)
   ADD_TEST_MACRO(MacroTest miniMacroTest)
   ADD_TEST_MACRO(FunctionTest miniFunctionTest)

Tests/LinkLanguage/CMakeLists.txt

@@ -0,0 +1,10 @@
+cmake_minimum_required(VERSION 2.7.20090708)
+project(LinkLanguage C CXX)
+
+add_library(foo STATIC foo.cxx)
+add_executable(LinkLanguage LinkLanguage.c)
+target_link_libraries(LinkLanguage foo)
+
+# CMake should now automatically choose CXX for linking, so we need
+# not set the property:
+#set_property(TARGET LinkLanguage PROPERTY LINKER_LANGUAGE CXX)

Tests/LinkLanguage/LinkLanguage.c

@@ -0,0 +1,6 @@
+extern int foo(void);
+int main()
+{
+  return foo();
+}
+

Tests/LinkLanguage/foo.cxx

@@ -0,0 +1,6 @@
+extern "C" int foo(void)
+{
+  // Reference C++ standard library symbols.
+  delete new int;
+  return 0;
+}