Let CMake decide when to start using constexpr

Sources/Plasma/CoreLib/HeadSpin.h

@@ -238,10 +238,12 @@ inline double hsSwapEndianDouble(double dvalue)
 *   StrPrintf(buffer, arrsize(buffer), "%u", value);
 *
 ***/
-#define  arrsize(a)     (sizeof(a) / sizeof((a)[0]))
-/* TODO: Use this safer version when MSVC finally supports constexpr */
-//template <typename _T, size_t _Sz>
-//constexpr size_t arrsize(_T (&)[_Sz]) { return _Sz; }
+#ifdef HAVE_CONSTEXPR
+    template <typename _T, size_t _Sz>
+    constexpr size_t arrsize(_T(&)[_Sz]) { return _Sz; }
+#else
+#   define  arrsize(a)  (sizeof(a) / sizeof((a)[0]))
+#endif
 
 
 /****************************************************************************

cmake/CompilerChecks.cmake

@@ -51,6 +51,11 @@ try_compile(HAVE_GCC_DEPRECATED_ATTR ${PROJECT_BINARY_DIR}
             COMPILE_DEFINITIONS -DTRY_GCC_ATTR
             OUTPUT_VARIABLE OUTPUT)
 
+# Look for C++11 constexpr support
+try_compile(HAVE_CONSTEXPR ${PROJECT_BINARY_DIR}
+            ${PROJECT_SOURCE_DIR}/cmake/check_constexpr.cpp
+            OUTPUT_VARIABLE OUTPUT)
+
 configure_file(${PROJECT_SOURCE_DIR}/cmake/hsCompilerSpecific.h.cmake
                ${PROJECT_BINARY_DIR}/hsCompilerSpecific.h)
 include_directories(${PROJECT_BINARY_DIR})

cmake/check_constexpr.cpp

@@ -0,0 +1,6 @@
+constexpr int foo() { return 5; }
+
+int main(int, char **)
+{
+    return foo();
+}

cmake/hsCompilerSpecific.h.cmake

@@ -51,4 +51,6 @@ You can contact Cyan Worlds, Inc. by email legal@cyan.com
 #cmakedefine HAVE_CXX14_DEPRECATED_ATTR
 #cmakedefine HAVE_GCC_DEPRECATED_ATTR
 
+#cmakedefine HAVE_CONSTEXPR
+
 #endif