[CMake] Add check for HAVE_CONSTEXPR.

cmake/modules/CheckCXX11Features.cmake

@@ -324,4 +324,22 @@ check_cxx_source_compiles("
 " HAVE_INITIALIZER_LIST
 )
 
+# constexpr
+check_cxx_source_compiles("
+  constexpr int foo()
+  { return 0; }
+
+  template<int v>
+  struct A
+  {
+    static const int value = v;
+  };
+
+  int main(void)
+  {
+    return A<foo()>::value;
+  }
+" HAVE_CONSTEXPR
+)
+
 cmake_pop_check_state()

config.h.cmake

@@ -160,6 +160,9 @@
 /* Define to 1 if initializer list is supported */
 #cmakedefine HAVE_INITIALIZER_LIST 1
 
+/* Define to 1 if C++11 constexpr is supported */
+#cmakedefine HAVE_CONSTEXPR 1
+
 /* Define to if the UMFPack library is available */
 #cmakedefine HAVE_UMFPACK ENABLE_UMFPACK