Fix RUBY_API_VERSION_NAME fallback definition

Author: nobu

version.c

@@ -31,13 +31,15 @@
 #  endif
 #  define RUBY_REVISION_STR " "RUBY_BRANCH_NAME" "RUBY_REVISION
 # else
-#  define RUBY_API_VERSION_NAME RUBY_API_VERSION_STR
 #  define RUBY_REVISION_STR " revision "RUBY_REVISION
 # endif
 #else
 # define RUBY_REVISION "HEAD"
 # define RUBY_REVISION_STR ""
 #endif
+#ifndef RUBY_API_VERSION_NAME
+# define RUBY_API_VERSION_NAME RUBY_API_VERSION_STR
+#endif
 #if !defined RUBY_RELEASE_DATETIME || RUBY_PATCHLEVEL != -1
 # undef RUBY_RELEASE_DATETIME
 # define RUBY_RELEASE_DATETIME RUBY_RELEASE_DATE