Allow external definition of MRB_API macro

When building WebAssembly targets with clang (e.g. wasm32-unknown-unknown), all
symbols are hidden by default. To export a symbol, it must be marked with

    __attribute__((visibility(default)))

Rather than detecting this exotic target in mruby, allow MRB_API to be defined
externally.

include/mruby/common.h

@@ -62,6 +62,7 @@ MRB_BEGIN_DECL
 #define MRB_INLINE static inline
 
 /** Declare a public MRuby API function. */
+#ifndef MRB_API
 #if defined(MRB_BUILD_AS_DLL)
 #if defined(MRB_CORE) || defined(MRB_LIB)
 # define MRB_API __declspec(dllexport)
@@ -71,6 +72,7 @@ MRB_BEGIN_DECL
 #else
 # define MRB_API extern
 #endif
+#endif
 
 MRB_END_DECL