try to link symlink and use a define on error

fixes compiling with Minimalist GNU win32 (cross) compiler 4.2.1, which
defines symlink, but doesn't implement it.

configure.ac

@@ -63,6 +63,7 @@ AC_SEARCH_LIBS(getopt, iberty)
 AC_CHECK_FUNCS(strtoimax strtoll)
 AC_CHECK_FUNCS(mergesort)
 AC_CHECK_FUNCS(mkstemps)
+AC_TRY_LINK_FUNC([symlink],[AC_DEFINE([HAVE_SYMLINK], 1, [Define to 1 if you have the symlink function.])])
 
 AC_OUTPUT(				\
 skeletons/standard-modules/Makefile	\

libasn1compiler/asn1c_save.c

@@ -6,6 +6,10 @@
 #include "asn1c_save.h"
 #include "asn1c_out.h"
 
+#ifndef HAVE_SYMLINK
+#define symlink(a,b) (errno=ENOSYS, -1)
+#endif
+
 #define	HINCLUDE(s)						\
 	((arg->flags & A1C_INCLUDES_QUOTED)			\
 		? fprintf(fp_h, "#include \"%s\"\n", s)		\