CXX: Compile when possible with explicit `Cxx` language flag set

Issue: #14516, #20716

Modules/Compiler/AppleClang-CXX.cmake

@@ -1,6 +1,8 @@
 include(Compiler/Clang)
 __compiler_clang(CXX)
 
+set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
+
 if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
   set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fvisibility-inlines-hidden")
 endif()

Modules/Compiler/Clang-CXX.cmake

@@ -2,7 +2,9 @@ include(Compiler/Clang)
 __compiler_clang(CXX)
 __compiler_clang_cxx_standards(CXX)
 
+
 if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
+  set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
   set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fvisibility-inlines-hidden")
 endif()
 

Modules/Compiler/GNU-CXX.cmake

@@ -1,6 +1,8 @@
 include(Compiler/GNU)
 __compiler_gnu(CXX)
 
+set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
+
 if (WIN32)
   if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.6)
     set(CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY_INLINES_HIDDEN "-fno-keep-inline-dllexport")

Modules/Compiler/Intel-CXX.cmake

@@ -37,6 +37,8 @@ if("x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
 
 else()
 
+  set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
+
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 18.0.0)
     set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++17")
     set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++17")

Modules/Compiler/XL-CXX.cmake

@@ -6,6 +6,8 @@ string(APPEND CMAKE_CXX_FLAGS_MINSIZEREL_INIT " -DNDEBUG")
 # -qthreaded = Ensures that all optimizations will be thread-safe
 string(APPEND CMAKE_CXX_FLAGS_INIT " -qthreaded")
 
+set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -+)
+
 if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
   if(CMAKE_SYSTEM MATCHES "Linux")
     set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "")
@@ -32,6 +34,3 @@ if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
 endif ()
 
 __compiler_check_default_language_standard(CXX 10.1 98)
-
-set(CMAKE_CXX_COMPILE_OBJECT
-  "<CMAKE_CXX_COMPILER> -+ <DEFINES> <INCLUDES> <FLAGS> -o <OBJECT> -c <SOURCE>")

Modules/Compiler/XLClang-CXX.cmake

@@ -1,6 +1,8 @@
 include(Compiler/XLClang)
 __compiler_xlclang(CXX)
 
+set(CMAKE_CXX_COMPILE_OPTIONS_EXPLICIT_LANGUAGE -x c++)
+
 if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 13.1.1)
   set(CMAKE_CXX98_STANDARD_COMPILE_OPTION  "")
   set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "")