Don't bail out if unsetenv is not found.

configure.ac

@@ -64,12 +64,13 @@ AC_PREFIX_DEFAULT([`pwd`])
 AC_CHECK_LIB([m],[main],,
              [AC_MSG_ERROR([no math library, exiting...!])] )
 
-AC_CHECK_FUNCS([alarm pow sqrt strdup strchr strstr unsetenv],,
+AC_CHECK_FUNCS([alarm pow sqrt strdup strchr strstr],,
 	       [AC_MSG_ERROR([Sorry, these are needed..])])
 
 AC_FUNC_MALLOC
 AC_FUNC_VPRINTF
 
+AC_CHECK_FUNCS([unsetenv])
 AC_CHECK_FUNCS([getopt_long],[],
 	[AC_LIBOBJ([getopt])
 	AC_LIBOBJ([getopt1])