OpenSSH has only been tested with GNU make. It may work with other
'make' programs, but you are on your own.
-GNU rx:
-ftp:://ftp.gnu.org/gnu/rx/
+pcre (POSIX Regular Expression library):
+ftp://ftp.cus.cam.ac.uk/pub/software/programs/pcre/
Most platforms do not required this. However older 4.3 BSD do not
have a posix regex library.