Tests/Module/WCDH: write multi_file_compiler_detection.h before using it

This was broken in commit 98e6d1e5e426c491e04faa746c11746002e6a69d
(Tests/Module/WCDH: make it work with only C features defined) when all C tests
were made accessible even if no C++ features are available, but the header was
only created if C++ features are available. Fix it by creating the header
unconditionally before any checks on the available features.

Tests/Module/WriteCompilerDetectionHeader/CMakeLists.txt

@@ -20,6 +20,17 @@ write_compiler_detection_header(
     ${cxx_known_features} ${c_known_features}
 )
 
+write_compiler_detection_header(
+  FILE "${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files/multi_file_compiler_detection.h"
+  PREFIX MULTI
+  OUTPUT_FILES_VAR multi_files
+  OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files/compiler_support"
+  COMPILERS GNU Clang AppleClang MSVC SunPro Intel
+  VERSION 3.1
+  FEATURES
+    ${cxx_known_features} ${c_known_features}
+)
+
 macro(set_defines target true_defs false_defs)
   set(defines)
   foreach(def ${true_defs})
@@ -116,17 +127,6 @@ add_executable(WriteCompilerDetectionHeader main.cpp)
 set_property(TARGET WriteCompilerDetectionHeader PROPERTY CXX_STANDARD 98)
 set_defines(WriteCompilerDetectionHeader "${true_defs}" "${false_defs}")
 
-write_compiler_detection_header(
-  FILE "${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files/multi_file_compiler_detection.h"
-  PREFIX MULTI
-  OUTPUT_FILES_VAR multi_files
-  OUTPUT_DIR "${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files/compiler_support"
-  COMPILERS GNU Clang AppleClang MSVC SunPro Intel
-  VERSION 3.1
-  FEATURES
-    ${cxx_known_features} ${c_known_features}
-)
-
 add_executable(multi_files multi_files.cpp)
 set_property(TARGET multi_files PROPERTY CXX_STANDARD 98)
 target_include_directories(multi_files PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/compiler_multi_files)