Fixes #393.

Changed the jas_config.h.in header template in an attempt to avoid
undesirable compiler warnings when JasPer is being used in C++ code.

Author: mdadams

src/libjasper/include/jasper/jas_config.h.in

@@ -113,7 +113,8 @@ The version of the C standard against which JasPer was built.
 #define JAS_DEFAULT_MAX_MEM_USAGE (1024ULL * 1024ULL * 1024ULL)
 #endif
 
-#if (__STDC_VERSION__ - 0 < JAS_STDC_VERSION)
+#if (!defined(__cplusplus) && (__STDC_VERSION__ - 0 < JAS_STDC_VERSION)) || \
+  (defined(__cplusplus) && (__cplusplus - 0 < JAS_STDC_VERSION))
 #warning "Your code is being built against an older version of the C standard than JasPer was.  Although this is supported, this may require some extra preprocessor defines when building."
 #endif