Choose compiler-specific flags earlier in CMake's own build

Add the flags early enough be used in the checks for C++ features.

CMakeLists.txt

@@ -63,6 +63,10 @@ if(NOT DEFINED CMAKE_CXX_STANDARD AND NOT CMake_NO_CXX_STANDARD)
   endif()
 endif()
 if(NOT CMake_TEST_EXTERNAL_CMAKE)
+  # include special compile flags for some compilers
+  include(CompileFlags.cmake)
+
+  # check for available C++ features
   include(${CMake_SOURCE_DIR}/Source/Checks/cm_cxx_features.cmake)
 endif()
 
@@ -648,9 +652,6 @@ if(NOT CMake_TEST_EXTERNAL_CMAKE)
     include(${CMake_SOURCE_DIR}/Tests/CMakeInstall.cmake)
   endif()
 
-  # include special compile flags for some compilers
-  include(CompileFlags.cmake)
-
   # no clue why we are testing for this here
   include(CheckSymbolExists)
   CHECK_SYMBOL_EXISTS(unsetenv "stdlib.h" HAVE_UNSETENV)