Features: Blacklist cxx_constexpr test for GNU 4.5.

Tests/CompileFeatures/CMakeLists.txt

@@ -51,6 +51,14 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang"
   )
 endif()
 
+if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
+    AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.6)
+  # The cxx_constexpr feature happens to work (for *this* testcase) with
+  # GNU 4.5, but it is first documented as available with GNU 4.6.
+  list(REMOVE_ITEM CXX_non_features
+    cxx_constexpr
+  )
+endif()
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
     AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
   # The cxx_alignof feature happens to work (for *this* testcase) with