GCC<=4.9: Exceptions (#76)

GCC 4.8 and 4.9 support exceptions, but are not detected as such.
This adds exception support for those compilers.

Detection taken from `nlohmann/json`.

Author: ax3l

include/mpark/config.hpp

@@ -64,7 +64,8 @@
 #endif
 
 #if __has_feature(cxx_exceptions) || defined(__cpp_exceptions) || \
-    (defined(_MSC_VER) && defined(_CPPUNWIND))
+    (defined(_MSC_VER) && defined(_CPPUNWIND)) || \
+    defined(__EXCEPTIONS)
 #define MPARK_EXCEPTIONS
 #endif